diff --git a/packages/react-cache/src/LRU.js b/packages/react-cache/src/LRU.js
index 8462849c4463b..60f6e6ea099a1 100644
--- a/packages/react-cache/src/LRU.js
+++ b/packages/react-cache/src/LRU.js
@@ -57,7 +57,7 @@ export function createLRU<T>(limit: number): LRU<T> {
     // Delete entries from the cache, starting from the end of the list.
     if (first !== null) {
       const resolvedFirst: Entry<T> = (first: any);
-      let last = resolvedFirst.previous;
+      let last: null | Entry<T> = resolvedFirst.previous;
       while (size > targetSize && last !== null) {
         const onDelete = last.onDelete;
         const previous = last.previous;
diff --git a/packages/react-debug-tools/src/ReactDebugHooks.js b/packages/react-debug-tools/src/ReactDebugHooks.js
index 6409647772eff..2bd458d06757d 100644
--- a/packages/react-debug-tools/src/ReactDebugHooks.js
+++ b/packages/react-debug-tools/src/ReactDebugHooks.js
@@ -217,7 +217,7 @@ function useImperativeHandle<T>(
   // and if there is a ref callback it might not store it but if it does we
   // have no way of knowing where. So let's only enable introspection of the
   // ref itself if it is using the object form.
-  let instance = undefined;
+  let instance: ?T = undefined;
   if (ref !== null && typeof ref === 'object') {
     instance = ref.current;
   }
@@ -716,6 +716,7 @@ export function inspectHooks<Props>(
   } finally {
     readHookLog = hookLog;
     hookLog = [];
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     currentDispatcher.current = previousDispatcher;
   }
   const rootStack = ErrorStackParser.parse(ancestorStackError);
@@ -723,7 +724,7 @@ export function inspectHooks<Props>(
 }
 
 function setupContexts(contextMap: Map<ReactContext<any>, any>, fiber: Fiber) {
-  let current = fiber;
+  let current: null | Fiber = fiber;
   while (current) {
     if (current.tag === ContextProvider) {
       const providerType: ReactProviderType<any> = current.type;
diff --git a/packages/react-devtools-core/src/backend.js b/packages/react-devtools-core/src/backend.js
index b63cd13006e3d..48a1348ee05cc 100644
--- a/packages/react-devtools-core/src/backend.js
+++ b/packages/react-devtools-core/src/backend.js
@@ -146,6 +146,7 @@ export function connectToDevTools(options: ?ConnectOptions) {
         }
       },
     );
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     bridge.addListener(
       'updateComponentFilters',
       (componentFilters: Array<ComponentFilter>) => {
@@ -165,10 +166,12 @@ export function connectToDevTools(options: ?ConnectOptions) {
     // Ideally the backend would save the filters itself, but RN doesn't provide a sync storage solution.
     // So for now we just fall back to using the default filters...
     if (window.__REACT_DEVTOOLS_COMPONENT_FILTERS__ == null) {
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       bridge.send('overrideComponentFilters', savedComponentFilters);
     }
 
     // TODO (npm-packages) Warn if "isBackendStorageAPISupported"
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     const agent = new Agent(bridge);
     agent.addListener('shutdown', () => {
       // If we received 'shutdown' from `agent`, we assume the `bridge` is already shutting down,
@@ -181,6 +184,7 @@ export function connectToDevTools(options: ?ConnectOptions) {
     // Setup React Native style editor if the environment supports it.
     if (resolveRNStyle != null || hook.resolveRNStyle != null) {
       setupNativeStyleEditor(
+        // $FlowFixMe[incompatible-call] found when upgrading Flow
         bridge,
         agent,
         ((resolveRNStyle || hook.resolveRNStyle: any): ResolveNativeStyle),
diff --git a/packages/react-devtools-core/src/standalone.js b/packages/react-devtools-core/src/standalone.js
index 315b6bcaf936e..6e1f403f9800d 100644
--- a/packages/react-devtools-core/src/standalone.js
+++ b/packages/react-devtools-core/src/standalone.js
@@ -256,6 +256,7 @@ function initialize(socket: WebSocket) {
     socket.close();
   });
 
+  // $FlowFixMe[incompatible-call] found when upgrading Flow
   store = new Store(bridge, {
     checkBridgeProtocolCompatibility: true,
     supportsNativeInspection: true,
diff --git a/packages/react-devtools-shared/src/backend/DevToolsFiberComponentStack.js b/packages/react-devtools-shared/src/backend/DevToolsFiberComponentStack.js
index 4bec30e92b45f..c1e1a2ed04b31 100644
--- a/packages/react-devtools-shared/src/backend/DevToolsFiberComponentStack.js
+++ b/packages/react-devtools-shared/src/backend/DevToolsFiberComponentStack.js
@@ -84,9 +84,10 @@ export function getStackByFiberInDevAndProd(
 ): string {
   try {
     let info = '';
-    let node = workInProgress;
+    let node: Fiber = workInProgress;
     do {
       info += describeFiber(workTagMap, node, currentDispatcherRef);
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       node = node.return;
     } while (node);
     return info;
diff --git a/packages/react-devtools-shared/src/backend/legacy/renderer.js b/packages/react-devtools-shared/src/backend/legacy/renderer.js
index 1f53b61d71e82..48cbe21f5bb08 100644
--- a/packages/react-devtools-shared/src/backend/legacy/renderer.js
+++ b/packages/react-devtools-shared/src/backend/legacy/renderer.js
@@ -248,6 +248,7 @@ export function attach(
           parentIDStack.pop();
           return result;
         } catch (err) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           parentIDStack = [];
           throw err;
         } finally {
@@ -284,6 +285,7 @@ export function attach(
           parentIDStack.pop();
           return result;
         } catch (err) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           parentIDStack = [];
           throw err;
         } finally {
@@ -320,6 +322,7 @@ export function attach(
           parentIDStack.pop();
           return result;
         } catch (err) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           parentIDStack = [];
           throw err;
         } finally {
@@ -351,6 +354,7 @@ export function attach(
 
           return result;
         } catch (err) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           parentIDStack = [];
           throw err;
         } finally {
diff --git a/packages/react-devtools-shared/src/backend/profilingHooks.js b/packages/react-devtools-shared/src/backend/profilingHooks.js
index a6c314584c140..c91bb36a16148 100644
--- a/packages/react-devtools-shared/src/backend/profilingHooks.js
+++ b/packages/react-devtools-shared/src/backend/profilingHooks.js
@@ -799,7 +799,7 @@ export function createProfilingHooks({
 
   function getParentFibers(fiber: Fiber): Array<Fiber> {
     const parents = [];
-    let parent = fiber;
+    let parent: null | Fiber = fiber;
     while (parent !== null) {
       parents.push(parent);
       parent = parent.return;
@@ -824,6 +824,7 @@ export function createProfilingHooks({
             warning: null,
           };
           currentFiberStacks.set(event, getParentFibers(fiber));
+          // $FlowFixMe[incompatible-use] found when upgrading Flow
           currentTimelineData.schedulingEvents.push(event);
         }
       }
diff --git a/packages/react-devtools-shared/src/backend/renderer.js b/packages/react-devtools-shared/src/backend/renderer.js
index c4ff00e08f423..4e9dbb3156c1c 100644
--- a/packages/react-devtools-shared/src/backend/renderer.js
+++ b/packages/react-devtools-shared/src/backend/renderer.js
@@ -2269,7 +2269,7 @@ export function attach(
 
     // This is a naive implementation that shallowly recourses children.
     // We might want to revisit this if it proves to be too inefficient.
-    let child = childSet;
+    let child: null | Fiber = childSet;
     while (child !== null) {
       findReorderedChildrenRecursively(child, nextChildren);
       child = child.sibling;
@@ -2846,11 +2846,11 @@ export function attach(
   // https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberTreeReflection.js
   function getNearestMountedFiber(fiber: Fiber): null | Fiber {
     let node = fiber;
-    let nearestMounted = fiber;
+    let nearestMounted: null | Fiber = fiber;
     if (!fiber.alternate) {
       // If there is no alternate, this might be a new tree that isn't inserted
       // yet. If it is, then it will have a pending insertion effect on it.
-      let nextNode = node;
+      let nextNode: Fiber = node;
       do {
         node = nextNode;
         if ((node.flags & (Placement | Hydrating)) !== NoFlags) {
@@ -2859,6 +2859,7 @@ export function attach(
           // if that one is still mounted.
           nearestMounted = node.return;
         }
+        // $FlowFixMe[incompatible-type] we bail out when we get a null
         nextNode = node.return;
       } while (nextNode);
     } else {
@@ -3097,7 +3098,7 @@ export function attach(
     const owners: Array<SerializedElement> = [fiberToSerializedElement(fiber)];
 
     if (_debugOwner) {
-      let owner = _debugOwner;
+      let owner: null | Fiber = _debugOwner;
       while (owner !== null) {
         owners.unshift(fiberToSerializedElement(owner));
         owner = owner._debugOwner || null;
@@ -3256,7 +3257,7 @@ export function attach(
     let owners = null;
     if (_debugOwner) {
       owners = [];
-      let owner = _debugOwner;
+      let owner: null | Fiber = _debugOwner;
       while (owner !== null) {
         owners.push(fiberToSerializedElement(owner));
         owner = owner._debugOwner || null;
@@ -3687,18 +3688,22 @@ export function attach(
     // This will enable us to send patches without re-inspecting if hydrated paths are requested.
     // (Reducing how often we shallow-render is a better DX for function components that use hooks.)
     const cleanedInspectedElement = {...mostRecentlyInspectedElement};
+    // $FlowFixMe[prop-missing] found when upgrading Flow
     cleanedInspectedElement.context = cleanForBridge(
       cleanedInspectedElement.context,
       createIsPathAllowed('context', null),
     );
+    // $FlowFixMe[prop-missing] found when upgrading Flow
     cleanedInspectedElement.hooks = cleanForBridge(
       cleanedInspectedElement.hooks,
       createIsPathAllowed('hooks', 'hooks'),
     );
+    // $FlowFixMe[prop-missing] found when upgrading Flow
     cleanedInspectedElement.props = cleanForBridge(
       cleanedInspectedElement.props,
       createIsPathAllowed('props', null),
     );
+    // $FlowFixMe[prop-missing] found when upgrading Flow
     cleanedInspectedElement.state = cleanForBridge(
       cleanedInspectedElement.state,
       createIsPathAllowed('state', null),
@@ -3709,6 +3714,7 @@ export function attach(
       responseID: requestID,
       type: 'full-data',
       // $FlowFixMe[incompatible-return] found when upgrading Flow
+      // $FlowFixMe[prop-missing] found when upgrading Flow
       value: cleanedInspectedElement,
     };
   }
@@ -4420,13 +4426,15 @@ export function attach(
   // The return path will contain Fibers that are "invisible" to the store
   // because their keys and indexes are important to restoring the selection.
   function getPathForElement(id: number): Array<PathFrame> | null {
-    let fiber = idToArbitraryFiberMap.get(id);
+    let fiber: ?Fiber = idToArbitraryFiberMap.get(id);
     if (fiber == null) {
       return null;
     }
     const keyPath = [];
     while (fiber !== null) {
+      // $FlowFixMe[incompatible-call] found when upgrading Flow
       keyPath.push(getPathFrame(fiber));
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       fiber = fiber.return;
     }
     keyPath.reverse();
@@ -4443,7 +4451,7 @@ export function attach(
       return null;
     }
     // Find the closest Fiber store is aware of.
-    let fiber = trackedPathMatchFiber;
+    let fiber: null | Fiber = trackedPathMatchFiber;
     while (fiber !== null && shouldFilterFiber(fiber)) {
       fiber = fiber.return;
     }
diff --git a/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js b/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
index 577aaf912860e..4457d67e19e13 100644
--- a/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
+++ b/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
@@ -282,7 +282,7 @@ function findTipPos(dims, bounds, tipSize) {
   const tipWidth = Math.max(tipSize.width, 60);
   const margin = 5;
 
-  let top;
+  let top: number | string;
   if (dims.top + dims.height + tipHeight <= bounds.top + bounds.height) {
     if (dims.top + dims.height < bounds.top + 0) {
       top = bounds.top + margin;
@@ -299,7 +299,7 @@ function findTipPos(dims, bounds, tipSize) {
     top = bounds.top + bounds.height - tipHeight - margin;
   }
 
-  let left = dims.left + margin;
+  let left: number | string = dims.left + margin;
   if (dims.left < bounds.left) {
     left = bounds.left + margin;
   }
diff --git a/packages/react-devtools-shared/src/backend/views/utils.js b/packages/react-devtools-shared/src/backend/views/utils.js
index 43c6ffdc197a2..2e61cf90acb16 100644
--- a/packages/react-devtools-shared/src/backend/views/utils.js
+++ b/packages/react-devtools-shared/src/backend/views/utils.js
@@ -84,7 +84,7 @@ export function getNestedBoundingClientRect(
   const ownerIframe = getOwnerIframe(node);
   if (ownerIframe && ownerIframe !== boundaryWindow) {
     const rects = [node.getBoundingClientRect()];
-    let currentIframe = ownerIframe;
+    let currentIframe: null | HTMLElement = ownerIframe;
     let onlyOneMore = false;
     while (currentIframe) {
       const rect = getBoundingClientRectWithBorderOffset(currentIframe);
diff --git a/packages/react-devtools-shared/src/devtools/cache.js b/packages/react-devtools-shared/src/devtools/cache.js
index 693153715bfc2..b0a01a443845a 100644
--- a/packages/react-devtools-shared/src/devtools/cache.js
+++ b/packages/react-devtools-shared/src/devtools/cache.js
@@ -88,7 +88,9 @@ const resourceConfigs: Map<Resource<any, any, any>, Config> = new Map();
 function getEntriesForResource(
   resource: any,
 ): Map<any, any> | WeakMap<any, any> {
-  let entriesForResource = ((entries.get(resource): any): Map<any, any>);
+  let entriesForResource: Map<any, any> | WeakMap<any, any> = ((entries.get(
+    resource,
+  ): any): Map<any, any>);
   if (entriesForResource === undefined) {
     const config = resourceConfigs.get(resource);
     entriesForResource =
diff --git a/packages/react-devtools-shared/src/devtools/store.js b/packages/react-devtools-shared/src/devtools/store.js
index 11d4a9a5f1f39..fb0c6d868fad4 100644
--- a/packages/react-devtools-shared/src/devtools/store.js
+++ b/packages/react-devtools-shared/src/devtools/store.js
@@ -763,7 +763,7 @@ export default class Store extends EventEmitter<{
 
           const weightDelta = 1 - element.weight;
 
-          let parentElement = ((this._idToElement.get(
+          let parentElement: void | Element = ((this._idToElement.get(
             element.parentID,
           ): any): Element);
           while (parentElement != null) {
@@ -789,7 +789,7 @@ export default class Store extends EventEmitter<{
               : currentElement.weight;
             const weightDelta = newWeight - oldWeight;
 
-            let parentElement = ((this._idToElement.get(
+            let parentElement: void | Element = ((this._idToElement.get(
               currentElement.parentID,
             ): any): Element);
             while (parentElement != null) {
@@ -806,8 +806,10 @@ export default class Store extends EventEmitter<{
 
           currentElement =
             currentElement.parentID !== 0
-              ? this.getElementByID(currentElement.parentID)
-              : null;
+              ? // $FlowFixMe[incompatible-type] found when upgrading Flow
+                this.getElementByID(currentElement.parentID)
+              : // $FlowFixMe[incompatible-type] found when upgrading Flow
+                null;
         }
       }
 
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/context.js b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/context.js
index 583ac1e70d837..1ce7ff078a743 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/context.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/context.js
@@ -66,7 +66,11 @@ const resource: Resource<
       return request.promise;
     }
 
-    let resolveFn = ((null: any): ResolveFn);
+    let resolveFn:
+      | ResolveFn
+      | ((
+          result: Promise<StyleAndLayoutFrontend> | StyleAndLayoutFrontend,
+        ) => void) = ((null: any): ResolveFn);
     const promise = new Promise(resolve => {
       resolveFn = resolve;
     });
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js b/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
index 04c141e6f6cf9..0190f7dece19e 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
@@ -57,7 +57,7 @@ export default function NewKeyValue({
     const {id} = inspectedElement;
     const rendererID = store.getRendererIDForElement(id);
     if (rendererID !== null) {
-      let basePath = newPath;
+      let basePath: Array<string | number> = newPath;
       if (hookID != null) {
         basePath = parseHookPathForEdit(basePath);
       }
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/OwnersListContext.js b/packages/react-devtools-shared/src/devtools/views/Components/OwnersListContext.js
index a6f929c46dacf..eadf2037f0510 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/OwnersListContext.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/OwnersListContext.js
@@ -49,7 +49,11 @@ const resource: Resource<
       return request.promise;
     }
 
-    let resolveFn = ((null: any): ResolveFn);
+    let resolveFn:
+      | ResolveFn
+      | ((
+          result: Promise<Array<SerializedElement>> | Array<SerializedElement>,
+        ) => void) = ((null: any): ResolveFn);
     const promise = new Promise(resolve => {
       resolveFn = resolve;
     });
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/SelectedTreeHighlight.js b/packages/react-devtools-shared/src/devtools/views/Components/SelectedTreeHighlight.js
index 05b34ab66ed7e..08e7f51b557ec 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/SelectedTreeHighlight.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/SelectedTreeHighlight.js
@@ -7,6 +7,7 @@
  * @flow
  */
 
+import type {Element} from './types';
 import * as React from 'react';
 import {useContext, useMemo} from 'react';
 import {TreeStateContext} from './TreeContext';
@@ -53,7 +54,7 @@ export default function SelectedTreeHighlight(_: {}): React.Node {
         }
 
         let stopIndex = null;
-        let current = element;
+        let current: null | Element = element;
         while (current !== null) {
           if (current.isCollapsed || current.children.length === 0) {
             // We've found the last/deepest descendant.
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js b/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
index 90208c5bae387..027bf1d652f14 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/TreeContext.js
@@ -294,7 +294,7 @@ function reduceTreeState(store: Store, state: State, action: Action): State {
           ) {
             const leafElement = store.getElementByID(ownerSubtreeLeafElementID);
             if (leafElement !== null) {
-              let currentElement = leafElement;
+              let currentElement: null | Element = leafElement;
               while (currentElement !== null) {
                 if (currentElement.ownerID === selectedElementID) {
                   selectedElementIndex = store.getIndexOfElementID(
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js b/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
index 5287cf3d56308..baf61f82822be 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
@@ -57,8 +57,8 @@ const TOOLTIP_OFFSET = 5;
 // Method used to find the position of the tooltip based on current mouse position
 function getTooltipPosition(element, mousePosition) {
   const {height, mouseX, mouseY, width} = mousePosition;
-  let top = 0;
-  let left = 0;
+  let top: number | string = 0;
+  let left: number | string = 0;
 
   if (mouseY + TOOLTIP_OFFSET + element.offsetHeight >= height) {
     if (mouseY - TOOLTIP_OFFSET - element.offsetHeight > 0) {
diff --git a/packages/react-devtools-shared/src/devtools/views/hooks.js b/packages/react-devtools-shared/src/devtools/views/hooks.js
index 8366a221611fe..3baf29d0f7e4c 100644
--- a/packages/react-devtools-shared/src/devtools/views/hooks.js
+++ b/packages/react-devtools-shared/src/devtools/views/hooks.js
@@ -238,7 +238,7 @@ export function useModalDismissSignal(
     // Delay until after the current call stack is empty,
     // in case this effect is being run while an event is currently bubbling.
     // In that case, we don't want to listen to the pre-existing event.
-    let timeoutID = setTimeout(() => {
+    let timeoutID: null | TimeoutID = setTimeout(() => {
       timeoutID = null;
 
       // It's important to listen to the ownerDocument to support the browser extension.
diff --git a/packages/react-devtools-shared/src/dynamicImportCache.js b/packages/react-devtools-shared/src/dynamicImportCache.js
index 3e6c2f262837a..df55475add70c 100644
--- a/packages/react-devtools-shared/src/dynamicImportCache.js
+++ b/packages/react-devtools-shared/src/dynamicImportCache.js
@@ -136,7 +136,7 @@ export function loadModule(moduleLoaderFunction: ModuleLoaderFunction): Module {
     );
 
     // Eventually timeout and stop trying to load the module.
-    let timeoutID = setTimeout(function onTimeout() {
+    let timeoutID: null | TimeoutID = setTimeout(function onTimeout() {
       if (__DEBUG__) {
         console.log(
           `[dynamicImportCache] loadModule("${moduleLoaderFunction.name}") onTimeout()`,
diff --git a/packages/react-devtools-shared/src/registerDevToolsEventLogger.js b/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
index 4b044cf086119..72e5a635eab3c 100644
--- a/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
+++ b/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
@@ -13,7 +13,7 @@ import {registerEventLogger} from 'react-devtools-shared/src/Logger';
 import {enableLogger} from 'react-devtools-feature-flags';
 
 let loggingIFrame = null;
-let missedEvents = [];
+let missedEvents: Array<LoggerEvent> = [];
 
 type LoggerContext = {
   page_url: ?string,
diff --git a/packages/react-devtools-timeline/src/view-base/useCanvasInteraction.js b/packages/react-devtools-timeline/src/view-base/useCanvasInteraction.js
index 367569e65a6ab..628debee34b7c 100644
--- a/packages/react-devtools-timeline/src/view-base/useCanvasInteraction.js
+++ b/packages/react-devtools-timeline/src/view-base/useCanvasInteraction.js
@@ -125,6 +125,7 @@ export function useCanvasInteraction(
     }
 
     function localToCanvasCoordinates(localCoordinates: Point): Point {
+      // $FlowFixMe[incompatible-call] found when upgrading Flow
       const canvasRect = cacheFirstGetCanvasBoundingRect(canvas);
       return {
         x: localCoordinates.x - canvasRect.left,
diff --git a/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js b/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
index 22a8a17fadf31..500cdfb79f9c8 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
@@ -595,7 +595,7 @@ function createPreloadResource(
   const limitedEscapedHref = escapeSelectorAttributeValueInsideDoubleQuotes(
     href,
   );
-  let element = ownerDocument.querySelector(
+  let element: null | Instance | HTMLElement = ownerDocument.querySelector(
     `link[rel="preload"][href="${limitedEscapedHref}"]`,
   );
   if (!element) {
diff --git a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
index a6c193e17edbe..63c854a92acdd 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
@@ -593,7 +593,7 @@ export function clearSuspenseBoundary(
   parentInstance: Instance,
   suspenseInstance: SuspenseInstance,
 ): void {
-  let node = suspenseInstance;
+  let node: Node = suspenseInstance;
   // Delete all nodes within this suspense boundary.
   // There might be nested nodes so we need to keep track of how
   // deep we are and only break out when we're back on top.
@@ -620,6 +620,7 @@ export function clearSuspenseBoundary(
         depth++;
       }
     }
+    // $FlowFixMe[incompatible-type] we bail out when we get a null
     node = nextNode;
   } while (node);
   // TODO: Warn, we didn't find the end comment boundary.
diff --git a/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js b/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
index 131578fae8b6e..60565a7af8d71 100644
--- a/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
+++ b/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
@@ -427,7 +427,7 @@ function addTrappedEventListener(
   );
   // If passive option is not supported, then the event will be
   // active and not passive.
-  let isPassiveListener = undefined;
+  let isPassiveListener: void | boolean = undefined;
   if (passiveBrowserEventsSupported) {
     // Browsers introduced an intervention, making these events
     // passive by default on document. React doesn't bind them
@@ -578,7 +578,7 @@ export function dispatchEventForPluginEventSystem(
       // root boundaries that match that of our current "rootContainer".
       // If we find that "rootContainer", we find the parent fiber
       // sub-tree for that root and make that our ancestor instance.
-      let node = targetInst;
+      let node: null | Fiber = targetInst;
 
       mainLoop: while (true) {
         if (node === null) {
@@ -823,6 +823,7 @@ function getParent(inst: Fiber | null): Fiber | null {
     return null;
   }
   do {
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     inst = inst.return;
     // TODO: If this is a HostRoot we might want to bail out.
     // That is depending on if we want nested subtrees (layers) to bubble
@@ -841,14 +842,14 @@ function getParent(inst: Fiber | null): Fiber | null {
  * different trees.
  */
 function getLowestCommonAncestor(instA: Fiber, instB: Fiber): Fiber | null {
-  let nodeA = instA;
-  let nodeB = instB;
+  let nodeA: null | Fiber = instA;
+  let nodeB: null | Fiber = instB;
   let depthA = 0;
-  for (let tempA = nodeA; tempA; tempA = getParent(tempA)) {
+  for (let tempA: null | Fiber = nodeA; tempA; tempA = getParent(tempA)) {
     depthA++;
   }
   let depthB = 0;
-  for (let tempB = nodeB; tempB; tempB = getParent(tempB)) {
+  for (let tempB: null | Fiber = nodeB; tempB; tempB = getParent(tempB)) {
     depthB++;
   }
 
@@ -886,7 +887,7 @@ function accumulateEnterLeaveListenersForEvent(
   const registrationName = event._reactName;
   const listeners: Array<DispatchListener> = [];
 
-  let instance = target;
+  let instance: null | Fiber = target;
   while (instance !== null) {
     if (instance === common) {
       break;
diff --git a/packages/react-fs/src/ReactFilesystem.js b/packages/react-fs/src/ReactFilesystem.js
index da8d43c535243..2d376a4dca940 100644
--- a/packages/react-fs/src/ReactFilesystem.js
+++ b/packages/react-fs/src/ReactFilesystem.js
@@ -153,6 +153,7 @@ export function lstat(path: string, options?: {bigint?: boolean}): mixed {
   if (!record) {
     const thenable = fs.lstat(path, {bigint});
     record = createRecordFromThenable(thenable);
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     lstatCache.push(bigint, record);
   }
   // $FlowFixMe[incompatible-call] found when upgrading Flow
@@ -203,6 +204,7 @@ export function readdir(
   if (!record) {
     const thenable = fs.readdir(path, {encoding, withFileTypes});
     record = createRecordFromThenable(thenable);
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     readdirCache.push(encoding, withFileTypes, record);
   }
   // $FlowFixMe[incompatible-call] found when upgrading Flow
@@ -302,6 +304,7 @@ export function readlink(
   if (!record) {
     const thenable = fs.readlink(path, {encoding});
     record = createRecordFromThenable(thenable);
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     readlinkCache.push(encoding, record);
   }
   // $FlowFixMe[incompatible-call] found when upgrading Flow
@@ -344,6 +347,7 @@ export function realpath(
   if (!record) {
     const thenable = fs.realpath(path, {encoding});
     record = createRecordFromThenable(thenable);
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     realpathCache.push(encoding, record);
   }
   // $FlowFixMe[incompatible-call] found when upgrading Flow
@@ -379,6 +383,7 @@ export function stat(path: string, options?: {bigint?: boolean}): mixed {
   if (!record) {
     const thenable = fs.stat(path, {bigint});
     record = createRecordFromThenable(thenable);
+    // $FlowFixMe[incompatible-call] found when upgrading Flow
     statCache.push(bigint, record);
   }
   // $FlowFixMe[incompatible-call] found when upgrading Flow
diff --git a/packages/react-native-renderer/src/ReactNativeAttributePayload.js b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
index effdf12bace1e..64d73a503b273 100644
--- a/packages/react-native-renderer/src/ReactNativeAttributePayload.js
+++ b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
@@ -64,6 +64,7 @@ function restoreDeletedValuesInNestedArray(
   } else if (node && removedKeyCount > 0) {
     const obj = node;
     for (const propKey in removedKeys) {
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       if (!removedKeys[propKey]) {
         continue;
       }
@@ -78,9 +79,11 @@ function restoreDeletedValuesInNestedArray(
       }
 
       if (typeof nextProp === 'function') {
+        // $FlowFixMe[incompatible-type] found when upgrading Flow
         nextProp = true;
       }
       if (typeof nextProp === 'undefined') {
+        // $FlowFixMe[incompatible-type] found when upgrading Flow
         nextProp = null;
       }
 
diff --git a/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js b/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
index 7944c120a7294..eb12251a3e8a0 100644
--- a/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
+++ b/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
@@ -42,6 +42,7 @@ function recomputePluginOrdering(): void {
   }
   for (const pluginName in namesToPlugins) {
     const pluginModule = namesToPlugins[pluginName];
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     const pluginIndex = eventPluginOrder.indexOf(pluginName);
 
     if (pluginIndex <= -1) {
diff --git a/packages/react-pg/src/ReactPostgres.js b/packages/react-pg/src/ReactPostgres.js
index 1bad8df2aae4a..46426817ee43f 100644
--- a/packages/react-pg/src/ReactPostgres.js
+++ b/packages/react-pg/src/ReactPostgres.js
@@ -82,7 +82,7 @@ Pool.prototype.query = function(query: string, values?: Array<mixed>) {
   const outerMap = unstable_getCacheForType(this.createRecordMap);
 
   let innerMap: NestedMap = outerMap;
-  let key = query;
+  let key: mixed = query;
   if (values != null) {
     // If we have parameters, each becomes as a nesting layer for Maps.
     // We want to find (or create as needed) the innermost Map, and return that.
diff --git a/packages/react-reconciler/src/ReactChildFiber.new.js b/packages/react-reconciler/src/ReactChildFiber.new.js
index 179310ec46a91..a7b2d3e949797 100644
--- a/packages/react-reconciler/src/ReactChildFiber.new.js
+++ b/packages/react-reconciler/src/ReactChildFiber.new.js
@@ -319,7 +319,7 @@ function createChildReconciler(shouldTrackSideEffects): ChildReconciler {
     // instead.
     const existingChildren: Map<string | number, Fiber> = new Map();
 
-    let existingChild = currentFirstChild;
+    let existingChild: null | Fiber = currentFirstChild;
     while (existingChild !== null) {
       if (existingChild.key !== null) {
         existingChildren.set(existingChild.key, existingChild);
diff --git a/packages/react-reconciler/src/ReactChildFiber.old.js b/packages/react-reconciler/src/ReactChildFiber.old.js
index 74abb553458f7..4c880a3e5283d 100644
--- a/packages/react-reconciler/src/ReactChildFiber.old.js
+++ b/packages/react-reconciler/src/ReactChildFiber.old.js
@@ -319,7 +319,7 @@ function createChildReconciler(shouldTrackSideEffects): ChildReconciler {
     // instead.
     const existingChildren: Map<string | number, Fiber> = new Map();
 
-    let existingChild = currentFirstChild;
+    let existingChild: null | Fiber = currentFirstChild;
     while (existingChild !== null) {
       if (existingChild.key !== null) {
         existingChildren.set(existingChild.key, existingChild);
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.new.js b/packages/react-reconciler/src/ReactFiberBeginWork.new.js
index 3ba2334894d88..9e9c4bd9700d0 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.new.js
@@ -592,6 +592,7 @@ function updateSimpleMemoComponent(
         try {
           outerMemoType = init(payload);
         } catch (x) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           outerMemoType = null;
         }
         // Inner propTypes will be validated in the function component path.
@@ -2990,12 +2991,15 @@ function propagateSuspenseContextChange(
     if (node === workInProgress) {
       return;
     }
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     while (node.sibling === null) {
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       if (node.return === null || node.return === workInProgress) {
         return;
       }
       node = node.return;
     }
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     node.sibling.return = node.return;
     node = node.sibling;
   }
@@ -3596,13 +3600,16 @@ function remountFiber(
         // eslint-disable-next-line react-internal/prod-error-codes
         throw new Error('Expected parent to have a child.');
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (prevSibling.sibling !== oldWorkInProgress) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         prevSibling = prevSibling.sibling;
         if (prevSibling === null) {
           // eslint-disable-next-line react-internal/prod-error-codes
           throw new Error('Expected to find the previous sibling.');
         }
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       prevSibling.sibling = newWorkInProgress;
     }
 
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.old.js b/packages/react-reconciler/src/ReactFiberBeginWork.old.js
index f654dce025175..bd12a989801f0 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.old.js
@@ -592,6 +592,7 @@ function updateSimpleMemoComponent(
         try {
           outerMemoType = init(payload);
         } catch (x) {
+          // $FlowFixMe[incompatible-type] found when upgrading Flow
           outerMemoType = null;
         }
         // Inner propTypes will be validated in the function component path.
@@ -2990,12 +2991,15 @@ function propagateSuspenseContextChange(
     if (node === workInProgress) {
       return;
     }
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     while (node.sibling === null) {
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       if (node.return === null || node.return === workInProgress) {
         return;
       }
       node = node.return;
     }
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     node.sibling.return = node.return;
     node = node.sibling;
   }
@@ -3596,13 +3600,16 @@ function remountFiber(
         // eslint-disable-next-line react-internal/prod-error-codes
         throw new Error('Expected parent to have a child.');
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (prevSibling.sibling !== oldWorkInProgress) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         prevSibling = prevSibling.sibling;
         if (prevSibling === null) {
           // eslint-disable-next-line react-internal/prod-error-codes
           throw new Error('Expected to find the previous sibling.');
         }
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       prevSibling.sibling = newWorkInProgress;
     }
 
diff --git a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
index 6b76bd9453223..3688599a173cd 100644
--- a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
+++ b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.new.js
@@ -325,7 +325,7 @@ export function enqueueCapturedUpdate<State>(
       const firstBaseUpdate = queue.firstBaseUpdate;
       if (firstBaseUpdate !== null) {
         // Loop through the updates and clone them.
-        let update = firstBaseUpdate;
+        let update: Update<State> = firstBaseUpdate;
         do {
           const clone: Update<State> = {
             eventTime: update.eventTime,
@@ -345,6 +345,7 @@ export function enqueueCapturedUpdate<State>(
             newLast.next = clone;
             newLast = clone;
           }
+          // $FlowFixMe[incompatible-type] we bail out when we get a null
           update = update.next;
         } while (update !== null);
 
@@ -534,7 +535,7 @@ export function processUpdateQueue<State>(
     let newFirstBaseUpdate = null;
     let newLastBaseUpdate = null;
 
-    let update = firstBaseUpdate;
+    let update: Update<State> = firstBaseUpdate;
     do {
       // TODO: Don't need this field anymore
       const updateEventTime = update.eventTime;
@@ -620,6 +621,7 @@ export function processUpdateQueue<State>(
           }
         }
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       update = update.next;
       if (update === null) {
         pendingQueue = queue.shared.pending;
diff --git a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
index 0a2d3c4e75117..c4a3bf603e6f1 100644
--- a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
+++ b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.old.js
@@ -325,7 +325,7 @@ export function enqueueCapturedUpdate<State>(
       const firstBaseUpdate = queue.firstBaseUpdate;
       if (firstBaseUpdate !== null) {
         // Loop through the updates and clone them.
-        let update = firstBaseUpdate;
+        let update: Update<State> = firstBaseUpdate;
         do {
           const clone: Update<State> = {
             eventTime: update.eventTime,
@@ -345,6 +345,7 @@ export function enqueueCapturedUpdate<State>(
             newLast.next = clone;
             newLast = clone;
           }
+          // $FlowFixMe[incompatible-type] we bail out when we get a null
           update = update.next;
         } while (update !== null);
 
@@ -534,7 +535,7 @@ export function processUpdateQueue<State>(
     let newFirstBaseUpdate = null;
     let newLastBaseUpdate = null;
 
-    let update = firstBaseUpdate;
+    let update: Update<State> = firstBaseUpdate;
     do {
       // TODO: Don't need this field anymore
       const updateEventTime = update.eventTime;
@@ -620,6 +621,7 @@ export function processUpdateQueue<State>(
           }
         }
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       update = update.next;
       if (update === null) {
         pendingQueue = queue.shared.pending;
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.new.js b/packages/react-reconciler/src/ReactFiberCommitWork.new.js
index 18d30effcb704..3a7a696c05c30 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.new.js
@@ -1316,7 +1316,7 @@ function abortParentMarkerTransitionsForDeletedFiber(
   if (enableTransitionTracing) {
     // Find all pending markers that are waiting on child suspense boundaries in the
     // aborted subtree and cancels them
-    let fiber = abortedFiber;
+    let fiber: null | Fiber = abortedFiber;
     while (fiber !== null) {
       switch (fiber.tag) {
         case TracingMarkerComponent:
@@ -1785,6 +1785,7 @@ function getHostSibling(fiber: Fiber): ?Instance {
         // last sibling.
         return null;
       }
+      // $FlowFixMe[incompatible-type] found when upgrading Flow
       node = node.return;
     }
     node.sibling.return = node.return;
@@ -1951,7 +1952,7 @@ function commitDeletionEffects(
     // TODO: Instead of searching up the fiber return path on every deletion, we
     // can track the nearest host component on the JS stack as we traverse the
     // tree during the commit phase. This would make insertions faster, too.
-    let parent = returnFiber;
+    let parent: null | Fiber = returnFiber;
     findParent: while (parent !== null) {
       switch (parent.tag) {
         case HostComponent: {
@@ -2332,7 +2333,10 @@ function getRetryCache(finishedWork) {
     }
     case OffscreenComponent: {
       const instance: OffscreenInstance = finishedWork.stateNode;
-      let retryCache = instance._retryCache;
+      // $FlowFixMe[incompatible-type-arg] found when upgrading Flow
+      let retryCache: null | Set<Wakeable> | WeakSet<Wakeable> =
+        // $FlowFixMe[incompatible-type] found when upgrading Flow
+        instance._retryCache;
       if (retryCache === null) {
         // $FlowFixMe[incompatible-type]
         retryCache = instance._retryCache = new PossiblyWeakSet();
@@ -3818,7 +3822,9 @@ function detachAlternateSiblings(parentFiber: Fiber) {
       if (detachedChild !== null) {
         previousFiber.child = null;
         do {
+          // $FlowFixMe[incompatible-use] found when upgrading Flow
           const detachedSibling = detachedChild.sibling;
+          // $FlowFixMe[incompatible-use] found when upgrading Flow
           detachedChild.sibling = null;
           detachedChild = detachedSibling;
         } while (detachedChild !== null);
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.old.js b/packages/react-reconciler/src/ReactFiberCommitWork.old.js
index fb2e9ab00e746..22c91330f1d20 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.old.js
@@ -1316,7 +1316,7 @@ function abortParentMarkerTransitionsForDeletedFiber(
   if (enableTransitionTracing) {
     // Find all pending markers that are waiting on child suspense boundaries in the
     // aborted subtree and cancels them
-    let fiber = abortedFiber;
+    let fiber: null | Fiber = abortedFiber;
     while (fiber !== null) {
       switch (fiber.tag) {
         case TracingMarkerComponent:
@@ -1785,6 +1785,7 @@ function getHostSibling(fiber: Fiber): ?Instance {
         // last sibling.
         return null;
       }
+      // $FlowFixMe[incompatible-type] found when upgrading Flow
       node = node.return;
     }
     node.sibling.return = node.return;
@@ -1951,7 +1952,7 @@ function commitDeletionEffects(
     // TODO: Instead of searching up the fiber return path on every deletion, we
     // can track the nearest host component on the JS stack as we traverse the
     // tree during the commit phase. This would make insertions faster, too.
-    let parent = returnFiber;
+    let parent: null | Fiber = returnFiber;
     findParent: while (parent !== null) {
       switch (parent.tag) {
         case HostComponent: {
@@ -2332,7 +2333,10 @@ function getRetryCache(finishedWork) {
     }
     case OffscreenComponent: {
       const instance: OffscreenInstance = finishedWork.stateNode;
-      let retryCache = instance._retryCache;
+      // $FlowFixMe[incompatible-type-arg] found when upgrading Flow
+      let retryCache: null | Set<Wakeable> | WeakSet<Wakeable> =
+        // $FlowFixMe[incompatible-type] found when upgrading Flow
+        instance._retryCache;
       if (retryCache === null) {
         // $FlowFixMe[incompatible-type]
         retryCache = instance._retryCache = new PossiblyWeakSet();
@@ -3818,7 +3822,9 @@ function detachAlternateSiblings(parentFiber: Fiber) {
       if (detachedChild !== null) {
         previousFiber.child = null;
         do {
+          // $FlowFixMe[incompatible-use] found when upgrading Flow
           const detachedSibling = detachedChild.sibling;
+          // $FlowFixMe[incompatible-use] found when upgrading Flow
           detachedChild.sibling = null;
           detachedChild = detachedSibling;
         } while (detachedChild !== null);
diff --git a/packages/react-reconciler/src/ReactFiberCompleteWork.new.js b/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
index eec23dfe83538..b2bdbb0e0bca6 100644
--- a/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
+++ b/packages/react-reconciler/src/ReactFiberCompleteWork.new.js
@@ -239,12 +239,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
@@ -359,12 +362,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
@@ -424,12 +430,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
diff --git a/packages/react-reconciler/src/ReactFiberCompleteWork.old.js b/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
index 10be55575c530..e0d1dc4b66325 100644
--- a/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
+++ b/packages/react-reconciler/src/ReactFiberCompleteWork.old.js
@@ -239,12 +239,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
@@ -359,12 +362,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
@@ -424,12 +430,15 @@ if (supportsMutation) {
       if (node === workInProgress) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       while (node.sibling === null) {
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         if (node.return === null || node.return === workInProgress) {
           return;
         }
         node = node.return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       node.sibling.return = node.return;
       node = node.sibling;
     }
diff --git a/packages/react-reconciler/src/ReactFiberComponentStack.js b/packages/react-reconciler/src/ReactFiberComponentStack.js
index 3821758c67a47..354aca57160e9 100644
--- a/packages/react-reconciler/src/ReactFiberComponentStack.js
+++ b/packages/react-reconciler/src/ReactFiberComponentStack.js
@@ -60,9 +60,10 @@ function describeFiber(fiber: Fiber): string {
 export function getStackByFiberInDevAndProd(workInProgress: Fiber): string {
   try {
     let info = '';
-    let node = workInProgress;
+    let node: Fiber = workInProgress;
     do {
       info += describeFiber(node);
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       node = node.return;
     } while (node);
     return info;
diff --git a/packages/react-reconciler/src/ReactFiberContext.new.js b/packages/react-reconciler/src/ReactFiberContext.new.js
index 23be350d21688..85dec63499bb9 100644
--- a/packages/react-reconciler/src/ReactFiberContext.new.js
+++ b/packages/react-reconciler/src/ReactFiberContext.new.js
@@ -303,7 +303,7 @@ function findCurrentUnmaskedContext(fiber: Fiber): Object {
       );
     }
 
-    let node = fiber;
+    let node: Fiber = fiber;
     do {
       switch (node.tag) {
         case HostRoot:
@@ -316,6 +316,7 @@ function findCurrentUnmaskedContext(fiber: Fiber): Object {
           break;
         }
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       node = node.return;
     } while (node !== null);
 
diff --git a/packages/react-reconciler/src/ReactFiberContext.old.js b/packages/react-reconciler/src/ReactFiberContext.old.js
index d4ba6c7c9f1c3..2e763a86c3f75 100644
--- a/packages/react-reconciler/src/ReactFiberContext.old.js
+++ b/packages/react-reconciler/src/ReactFiberContext.old.js
@@ -303,7 +303,7 @@ function findCurrentUnmaskedContext(fiber: Fiber): Object {
       );
     }
 
-    let node = fiber;
+    let node: Fiber = fiber;
     do {
       switch (node.tag) {
         case HostRoot:
@@ -316,6 +316,7 @@ function findCurrentUnmaskedContext(fiber: Fiber): Object {
           break;
         }
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       node = node.return;
     } while (node !== null);
 
diff --git a/packages/react-reconciler/src/ReactFiberHooks.new.js b/packages/react-reconciler/src/ReactFiberHooks.new.js
index 638f072150aa3..0c676de1a48da 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.new.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.new.js
@@ -387,7 +387,9 @@ function areHookInputsEqual(
       );
     }
   }
+  // $FlowFixMe[incompatible-use] found when upgrading Flow
   for (let i = 0; i < prevDeps.length && i < nextDeps.length; i++) {
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     if (is(nextDeps[i], prevDeps[i])) {
       continue;
     }
diff --git a/packages/react-reconciler/src/ReactFiberHooks.old.js b/packages/react-reconciler/src/ReactFiberHooks.old.js
index badad3a53bf6d..e0a1ee6070a30 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.old.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.old.js
@@ -387,7 +387,9 @@ function areHookInputsEqual(
       );
     }
   }
+  // $FlowFixMe[incompatible-use] found when upgrading Flow
   for (let i = 0; i < prevDeps.length && i < nextDeps.length; i++) {
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     if (is(nextDeps[i], prevDeps[i])) {
       continue;
     }
diff --git a/packages/react-reconciler/src/ReactFiberHotReloading.new.js b/packages/react-reconciler/src/ReactFiberHotReloading.new.js
index 223ba8088d8fa..0b226a3de9a61 100644
--- a/packages/react-reconciler/src/ReactFiberHotReloading.new.js
+++ b/packages/react-reconciler/src/ReactFiberHotReloading.new.js
@@ -303,6 +303,7 @@ function scheduleFibersWithFamiliesRecursively(
     if (failedBoundaries !== null) {
       if (
         failedBoundaries.has(fiber) ||
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         (alternate !== null && failedBoundaries.has(alternate))
       ) {
         needsRemount = true;
diff --git a/packages/react-reconciler/src/ReactFiberHotReloading.old.js b/packages/react-reconciler/src/ReactFiberHotReloading.old.js
index 0d4af2cad05e4..91d045a8944cb 100644
--- a/packages/react-reconciler/src/ReactFiberHotReloading.old.js
+++ b/packages/react-reconciler/src/ReactFiberHotReloading.old.js
@@ -303,6 +303,7 @@ function scheduleFibersWithFamiliesRecursively(
     if (failedBoundaries !== null) {
       if (
         failedBoundaries.has(fiber) ||
+        // $FlowFixMe[incompatible-use] found when upgrading Flow
         (alternate !== null && failedBoundaries.has(alternate))
       ) {
         needsRemount = true;
diff --git a/packages/react-reconciler/src/ReactFiberNewContext.new.js b/packages/react-reconciler/src/ReactFiberNewContext.new.js
index a311b101d3a4e..fc6432977793a 100644
--- a/packages/react-reconciler/src/ReactFiberNewContext.new.js
+++ b/packages/react-reconciler/src/ReactFiberNewContext.new.js
@@ -525,7 +525,7 @@ function propagateParentContextChanges(
   // Collect all the parent providers that changed. Since this is usually small
   // number, we use an Array instead of Set.
   let contexts = null;
-  let parent = workInProgress;
+  let parent: null | Fiber = workInProgress;
   let isInsidePropagationBailout = false;
   while (parent !== null) {
     if (!isInsidePropagationBailout) {
diff --git a/packages/react-reconciler/src/ReactFiberNewContext.old.js b/packages/react-reconciler/src/ReactFiberNewContext.old.js
index 0833d58806e9b..a39bd89354acd 100644
--- a/packages/react-reconciler/src/ReactFiberNewContext.old.js
+++ b/packages/react-reconciler/src/ReactFiberNewContext.old.js
@@ -525,7 +525,7 @@ function propagateParentContextChanges(
   // Collect all the parent providers that changed. Since this is usually small
   // number, we use an Array instead of Set.
   let contexts = null;
-  let parent = workInProgress;
+  let parent: null | Fiber = workInProgress;
   let isInsidePropagationBailout = false;
   while (parent !== null) {
     if (!isInsidePropagationBailout) {
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.new.js b/packages/react-reconciler/src/ReactFiberReconciler.new.js
index db77d28ff132b..08ded0097bbbf 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.new.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.new.js
@@ -525,7 +525,7 @@ export function findHostInstanceWithNoPortals(
   return hostFiber.stateNode;
 }
 
-let shouldErrorImpl = fiber => null;
+let shouldErrorImpl: Fiber => ?boolean = fiber => null;
 
 export function shouldError(fiber: Fiber): ?boolean {
   return shouldErrorImpl(fiber);
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.old.js b/packages/react-reconciler/src/ReactFiberReconciler.old.js
index aabd5d4b2ab3e..d71bf1c04baaa 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.old.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.old.js
@@ -525,7 +525,7 @@ export function findHostInstanceWithNoPortals(
   return hostFiber.stateNode;
 }
 
-let shouldErrorImpl = fiber => null;
+let shouldErrorImpl: Fiber => ?boolean = fiber => null;
 
 export function shouldError(fiber: Fiber): ?boolean {
   return shouldErrorImpl(fiber);
diff --git a/packages/react-reconciler/src/ReactFiberScope.new.js b/packages/react-reconciler/src/ReactFiberScope.new.js
index ccc589a9dfe60..7754cc6b54570 100644
--- a/packages/react-reconciler/src/ReactFiberScope.new.js
+++ b/packages/react-reconciler/src/ReactFiberScope.new.js
@@ -86,7 +86,7 @@ function collectScopedNodesFromChildren(
   fn: ReactScopeQuery,
   scopedNodes: Array<any>,
 ): void {
-  let child = startingChild;
+  let child: null | Fiber = startingChild;
   while (child !== null) {
     collectScopedNodes(child, fn, scopedNodes);
     child = child.sibling;
@@ -97,7 +97,7 @@ function collectFirstScopedNodeFromChildren(
   startingChild: Fiber,
   fn: ReactScopeQuery,
 ): Object | null {
-  let child = startingChild;
+  let child: null | Fiber = startingChild;
   while (child !== null) {
     const scopedNode = collectFirstScopedNode(child, fn);
     if (scopedNode !== null) {
diff --git a/packages/react-reconciler/src/ReactFiberScope.old.js b/packages/react-reconciler/src/ReactFiberScope.old.js
index ccc589a9dfe60..7754cc6b54570 100644
--- a/packages/react-reconciler/src/ReactFiberScope.old.js
+++ b/packages/react-reconciler/src/ReactFiberScope.old.js
@@ -86,7 +86,7 @@ function collectScopedNodesFromChildren(
   fn: ReactScopeQuery,
   scopedNodes: Array<any>,
 ): void {
-  let child = startingChild;
+  let child: null | Fiber = startingChild;
   while (child !== null) {
     collectScopedNodes(child, fn, scopedNodes);
     child = child.sibling;
@@ -97,7 +97,7 @@ function collectFirstScopedNodeFromChildren(
   startingChild: Fiber,
   fn: ReactScopeQuery,
 ): Object | null {
-  let child = startingChild;
+  let child: null | Fiber = startingChild;
   while (child !== null) {
     const scopedNode = collectFirstScopedNode(child, fn);
     if (scopedNode !== null) {
diff --git a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
index a1ef7d64bbf2e..2c86d0ae712d3 100644
--- a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
+++ b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.new.js
@@ -63,8 +63,9 @@ export function flushSyncCallbacks(): null {
       // $FlowFixMe[incompatible-use] found when upgrading Flow
       for (; i < queue.length; i++) {
         // $FlowFixMe[incompatible-use] found when upgrading Flow
-        let callback = queue[i];
+        let callback: SchedulerCallback = queue[i];
         do {
+          // $FlowFixMe[incompatible-type] we bail out when we get a null
           callback = callback(isSync);
         } while (callback !== null);
       }
diff --git a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
index 9286a9d156e6e..d914acf75f264 100644
--- a/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
+++ b/packages/react-reconciler/src/ReactFiberSyncTaskQueue.old.js
@@ -63,8 +63,9 @@ export function flushSyncCallbacks(): null {
       // $FlowFixMe[incompatible-use] found when upgrading Flow
       for (; i < queue.length; i++) {
         // $FlowFixMe[incompatible-use] found when upgrading Flow
-        let callback = queue[i];
+        let callback: SchedulerCallback = queue[i];
         do {
+          // $FlowFixMe[incompatible-type] we bail out when we get a null
           callback = callback(isSync);
         } while (callback !== null);
       }
diff --git a/packages/react-reconciler/src/ReactFiberThrow.new.js b/packages/react-reconciler/src/ReactFiberThrow.new.js
index d4e69b7c66940..b96df89fd54be 100644
--- a/packages/react-reconciler/src/ReactFiberThrow.new.js
+++ b/packages/react-reconciler/src/ReactFiberThrow.new.js
@@ -488,7 +488,7 @@ function throwException(
   // We didn't find a boundary that could handle this type of exception. Start
   // over and traverse parent path again, this time treating the exception
   // as an error.
-  let workInProgress = returnFiber;
+  let workInProgress: Fiber = returnFiber;
   do {
     switch (workInProgress.tag) {
       case HostRoot: {
@@ -528,6 +528,7 @@ function throwException(
       default:
         break;
     }
+    // $FlowFixMe[incompatible-type] we bail out when we get a null
     workInProgress = workInProgress.return;
   } while (workInProgress !== null);
 }
diff --git a/packages/react-reconciler/src/ReactFiberThrow.old.js b/packages/react-reconciler/src/ReactFiberThrow.old.js
index cdc7d3c2a79e4..c3ab61a5dc130 100644
--- a/packages/react-reconciler/src/ReactFiberThrow.old.js
+++ b/packages/react-reconciler/src/ReactFiberThrow.old.js
@@ -488,7 +488,7 @@ function throwException(
   // We didn't find a boundary that could handle this type of exception. Start
   // over and traverse parent path again, this time treating the exception
   // as an error.
-  let workInProgress = returnFiber;
+  let workInProgress: Fiber = returnFiber;
   do {
     switch (workInProgress.tag) {
       case HostRoot: {
@@ -528,6 +528,7 @@ function throwException(
       default:
         break;
     }
+    // $FlowFixMe[incompatible-type] we bail out when we get a null
     workInProgress = workInProgress.return;
   } while (workInProgress !== null);
 }
diff --git a/packages/react-reconciler/src/ReactFiberTreeReflection.js b/packages/react-reconciler/src/ReactFiberTreeReflection.js
index 67600492d13ff..da60614dfc6fb 100644
--- a/packages/react-reconciler/src/ReactFiberTreeReflection.js
+++ b/packages/react-reconciler/src/ReactFiberTreeReflection.js
@@ -30,11 +30,11 @@ const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
 
 export function getNearestMountedFiber(fiber: Fiber): null | Fiber {
   let node = fiber;
-  let nearestMounted = fiber;
+  let nearestMounted: null | Fiber = fiber;
   if (!fiber.alternate) {
     // If there is no alternate, this might be a new tree that isn't inserted
     // yet. If it is, then it will have a pending insertion effect on it.
-    let nextNode = node;
+    let nextNode: Fiber = node;
     do {
       node = nextNode;
       if ((node.flags & (Placement | Hydrating)) !== NoFlags) {
@@ -43,6 +43,7 @@ export function getNearestMountedFiber(fiber: Fiber): null | Fiber {
         // if that one is still mounted.
         nearestMounted = node.return;
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       nextNode = node.return;
     } while (nextNode);
   } else {
@@ -339,7 +340,7 @@ export function doesFiberContain(
   parentFiber: Fiber,
   childFiber: Fiber,
 ): boolean {
-  let node = childFiber;
+  let node: null | Fiber = childFiber;
   const parentFiberAlternate = parentFiber.alternate;
   while (node !== null) {
     if (node === parentFiber || node === parentFiberAlternate) {
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index 19a98052cd742..6cbe3fad2741a 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -742,7 +742,7 @@ export function scheduleUpdateOnFiber(
         root === rootCommittingMutationOrLayoutEffects
       ) {
         if (fiber.mode & ProfileMode) {
-          let current = fiber;
+          let current: null | Fiber = fiber;
           while (current !== null) {
             if (current.tag === Profiler) {
               const {id, onNestedUpdateScheduled} = current.memoizedProps;
@@ -2195,7 +2195,7 @@ function resumeSuspendedUnitOfWork(
 function completeUnitOfWork(unitOfWork: Fiber): void {
   // Attempt to complete the current unit of work, then move to the next
   // sibling. If there are no more siblings, return to the parent fiber.
-  let completedWork = unitOfWork;
+  let completedWork: Fiber = unitOfWork;
   do {
     // The current, flushed, state of this fiber is the alternate. Ideally
     // nothing should rely on this, but relying on it here means that we don't
@@ -2281,6 +2281,7 @@ function completeUnitOfWork(unitOfWork: Fiber): void {
       return;
     }
     // Otherwise, return to the parent
+    // $FlowFixMe[incompatible-type] we bail out when we get a null
     completedWork = returnFiber;
     // Update the next thing we're working on in case something throws.
     workInProgress = completedWork;
@@ -3388,7 +3389,7 @@ function invokeEffectsInDev(
   fiberFlags: Flags,
   invokeEffectFn: (fiber: Fiber) => void,
 ) {
-  let current = firstChild;
+  let current: null | Fiber = firstChild;
   let subtreeRoot = null;
   while (current != null) {
     const primarySubtreeFlag = current.subtreeFlags & fiberFlags;
@@ -3445,6 +3446,7 @@ export function warnAboutUpdateOnNotYetMountedFiberInDEV(fiber: Fiber) {
       if (didWarnStateUpdateForNotYetMountedComponent.has(componentName)) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       didWarnStateUpdateForNotYetMountedComponent.add(componentName);
     } else {
       didWarnStateUpdateForNotYetMountedComponent = new Set([componentName]);
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 6b8efdba30695..100b9c7b6f4e4 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -742,7 +742,7 @@ export function scheduleUpdateOnFiber(
         root === rootCommittingMutationOrLayoutEffects
       ) {
         if (fiber.mode & ProfileMode) {
-          let current = fiber;
+          let current: null | Fiber = fiber;
           while (current !== null) {
             if (current.tag === Profiler) {
               const {id, onNestedUpdateScheduled} = current.memoizedProps;
@@ -2195,7 +2195,7 @@ function resumeSuspendedUnitOfWork(
 function completeUnitOfWork(unitOfWork: Fiber): void {
   // Attempt to complete the current unit of work, then move to the next
   // sibling. If there are no more siblings, return to the parent fiber.
-  let completedWork = unitOfWork;
+  let completedWork: Fiber = unitOfWork;
   do {
     // The current, flushed, state of this fiber is the alternate. Ideally
     // nothing should rely on this, but relying on it here means that we don't
@@ -2281,6 +2281,7 @@ function completeUnitOfWork(unitOfWork: Fiber): void {
       return;
     }
     // Otherwise, return to the parent
+    // $FlowFixMe[incompatible-type] we bail out when we get a null
     completedWork = returnFiber;
     // Update the next thing we're working on in case something throws.
     workInProgress = completedWork;
@@ -3388,7 +3389,7 @@ function invokeEffectsInDev(
   fiberFlags: Flags,
   invokeEffectFn: (fiber: Fiber) => void,
 ) {
-  let current = firstChild;
+  let current: null | Fiber = firstChild;
   let subtreeRoot = null;
   while (current != null) {
     const primarySubtreeFlag = current.subtreeFlags & fiberFlags;
@@ -3445,6 +3446,7 @@ export function warnAboutUpdateOnNotYetMountedFiberInDEV(fiber: Fiber) {
       if (didWarnStateUpdateForNotYetMountedComponent.has(componentName)) {
         return;
       }
+      // $FlowFixMe[incompatible-use] found when upgrading Flow
       didWarnStateUpdateForNotYetMountedComponent.add(componentName);
     } else {
       didWarnStateUpdateForNotYetMountedComponent = new Set([componentName]);
diff --git a/packages/react-reconciler/src/ReactPostPaintCallback.js b/packages/react-reconciler/src/ReactPostPaintCallback.js
index a52623caa089b..aaa60db90c9dd 100644
--- a/packages/react-reconciler/src/ReactPostPaintCallback.js
+++ b/packages/react-reconciler/src/ReactPostPaintCallback.js
@@ -9,7 +9,7 @@
 import {requestPostPaintCallback} from './ReactFiberHostConfig';
 
 let postPaintCallbackScheduled = false;
-let callbacks = [];
+let callbacks: Array<any | ((endTime: number) => void)> = [];
 
 export function schedulePostPaintCallback(callback: (endTime: number) => void) {
   callbacks.push(callback);
diff --git a/packages/react-reconciler/src/ReactStrictModeWarnings.new.js b/packages/react-reconciler/src/ReactStrictModeWarnings.new.js
index c9e5ffce85cfb..41916797d0121 100644
--- a/packages/react-reconciler/src/ReactStrictModeWarnings.new.js
+++ b/packages/react-reconciler/src/ReactStrictModeWarnings.new.js
@@ -31,7 +31,7 @@ if (__DEV__) {
   const findStrictRoot = (fiber: Fiber): Fiber | null => {
     let maybeStrictRoot = null;
 
-    let node = fiber;
+    let node: null | Fiber = fiber;
     while (node !== null) {
       if (node.mode & StrictLegacyMode) {
         maybeStrictRoot = node;
diff --git a/packages/react-reconciler/src/ReactStrictModeWarnings.old.js b/packages/react-reconciler/src/ReactStrictModeWarnings.old.js
index c9e5ffce85cfb..41916797d0121 100644
--- a/packages/react-reconciler/src/ReactStrictModeWarnings.old.js
+++ b/packages/react-reconciler/src/ReactStrictModeWarnings.old.js
@@ -31,7 +31,7 @@ if (__DEV__) {
   const findStrictRoot = (fiber: Fiber): Fiber | null => {
     let maybeStrictRoot = null;
 
-    let node = fiber;
+    let node: null | Fiber = fiber;
     while (node !== null) {
       if (node.mode & StrictLegacyMode) {
         maybeStrictRoot = node;
diff --git a/packages/react-reconciler/src/ReactTestSelectors.js b/packages/react-reconciler/src/ReactTestSelectors.js
index 3ce6720c85a77..bda36f6d2a1aa 100644
--- a/packages/react-reconciler/src/ReactTestSelectors.js
+++ b/packages/react-reconciler/src/ReactTestSelectors.js
@@ -28,11 +28,11 @@ import {
   supportsTestSelectors,
 } from './ReactFiberHostConfig';
 
-let COMPONENT_TYPE = 0b000;
-let HAS_PSEUDO_CLASS_TYPE = 0b001;
-let ROLE_TYPE = 0b010;
-let TEST_NAME_TYPE = 0b011;
-let TEXT_TYPE = 0b100;
+let COMPONENT_TYPE: symbol | number = 0b000;
+let HAS_PSEUDO_CLASS_TYPE: symbol | number = 0b001;
+let ROLE_TYPE: symbol | number = 0b010;
+let TEST_NAME_TYPE: symbol | number = 0b011;
+let TEXT_TYPE: symbol | number = 0b100;
 
 if (typeof Symbol === 'function' && Symbol.for) {
   const symbolFor = Symbol.for;
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
index b7dcb25369899..1a64b305ff443 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
@@ -88,6 +88,7 @@ export default class ReactFlightWebpackPlugin {
     ) {
       this.clientReferences = [(options.clientReferences: $FlowFixMe)];
     } else {
+      // $FlowFixMe[incompatible-type] found when upgrading Flow
       this.clientReferences = options.clientReferences;
     }
     if (typeof options.chunkName === 'string') {
diff --git a/packages/react-server/src/ReactFizzComponentStack.js b/packages/react-server/src/ReactFizzComponentStack.js
index 1fa49453d31b2..cbdd6648336ad 100644
--- a/packages/react-server/src/ReactFizzComponentStack.js
+++ b/packages/react-server/src/ReactFizzComponentStack.js
@@ -39,7 +39,7 @@ export function getStackByComponentStackNode(
 ): string {
   try {
     let info = '';
-    let node = componentStack;
+    let node: ComponentStackNode = componentStack;
     do {
       switch (node.tag) {
         case 0:
@@ -52,6 +52,7 @@ export function getStackByComponentStackNode(
           info += describeClassComponentFrame(node.type, null, null);
           break;
       }
+      // $FlowFixMe[incompatible-type] we bail out when we get a null
       node = node.parent;
     } while (node);
     return info;
diff --git a/packages/react-server/src/ReactFizzHooks.js b/packages/react-server/src/ReactFizzHooks.js
index c2ed08aeb033e..d87c3a6daebf5 100644
--- a/packages/react-server/src/ReactFizzHooks.js
+++ b/packages/react-server/src/ReactFizzHooks.js
@@ -147,7 +147,9 @@ function areHookInputsEqual(
       );
     }
   }
+  // $FlowFixMe[incompatible-use] found when upgrading Flow
   for (let i = 0; i < prevDeps.length && i < nextDeps.length; i++) {
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     if (is(nextDeps[i], prevDeps[i])) {
       continue;
     }
@@ -344,7 +346,7 @@ export function useReducer<S, I, A>(
         renderPhaseUpdates.delete(queue);
         // $FlowFixMe[incompatible-use] found when upgrading Flow
         let newState = workInProgressHook.memoizedState;
-        let update = firstRenderPhaseUpdate;
+        let update: Update<any> = firstRenderPhaseUpdate;
         do {
           // Process this render phase update. We don't have to check the
           // priority because it will always be the same as the current
@@ -357,6 +359,7 @@ export function useReducer<S, I, A>(
           if (__DEV__) {
             isInHookUserCodeInDev = false;
           }
+          // $FlowFixMe[incompatible-type] we bail out when we get a null
           update = update.next;
         } while (update !== null);
 
@@ -442,6 +445,7 @@ function useRef<T>(initialValue: T): {current: T} {
     if (__DEV__) {
       Object.seal(ref);
     }
+    // $FlowFixMe[incompatible-use] found when upgrading Flow
     workInProgressHook.memoizedState = ref;
     return ref;
   } else {
diff --git a/packages/react-test-renderer/src/ReactTestRenderer.js b/packages/react-test-renderer/src/ReactTestRenderer.js
index 0b5947729bbe6..1bcd724eb7ec1 100644
--- a/packages/react-test-renderer/src/ReactTestRenderer.js
+++ b/packages/react-test-renderer/src/ReactTestRenderer.js
@@ -468,6 +468,7 @@ function create(
   let concurrentUpdatesByDefault = null;
   if (typeof options === 'object' && options !== null) {
     if (typeof options.createNodeMock === 'function') {
+      // $FlowFixMe[incompatible-type] found when upgrading Flow
       createNodeMock = options.createNodeMock;
     }
     if (options.unstable_isConcurrent === true) {
@@ -561,6 +562,7 @@ function create(
         return;
       }
       updateContainer(null, root, null, null);
+      // $FlowFixMe[incompatible-type] found when upgrading Flow
       container = null;
       root = null;
     },
diff --git a/packages/react/src/ReactAct.js b/packages/react/src/ReactAct.js
index 6a3712ef0b7f1..b4846205eb927 100644
--- a/packages/react/src/ReactAct.js
+++ b/packages/react/src/ReactAct.js
@@ -207,6 +207,7 @@ function flushActQueue(queue) {
         for (; i < queue.length; i++) {
           let callback = queue[i];
           do {
+            // $FlowFixMe[incompatible-type] found when upgrading Flow
             callback = callback(true);
           } while (callback !== null);
         }
diff --git a/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js b/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
index c7012e615ecd1..b61edccaac414 100644
--- a/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
+++ b/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
@@ -43,7 +43,7 @@ export function useSyncExternalStoreWithSelector<Snapshot, Selection>(
     // copies of the hook/component.
     let hasMemo = false;
     let memoizedSnapshot;
-    let memoizedSelection;
+    let memoizedSelection: Selection;
     const memoizedSelector = nextSnapshot => {
       if (!hasMemo) {
         // The first time the hook is called, there is no memoized result.
diff --git a/scripts/flow/config/flowconfig b/scripts/flow/config/flowconfig
index 7e0724cdcec47..92d91ac8a086a 100644
--- a/scripts/flow/config/flowconfig
+++ b/scripts/flow/config/flowconfig
@@ -43,7 +43,6 @@ untyped-type-import=error
 %CI_MAX_WORKERS%
 exact_by_default=true
 munge_underscores=false
-inference_mode=classic
 
 # Substituted by createFlowConfig.js:
 %REACT_RENDERER_FLOW_OPTIONS%