diff --git a/examples/simple/src/App.tsx b/examples/simple/src/App.tsx
index d9715949..a55b4f1f 100644
--- a/examples/simple/src/App.tsx
+++ b/examples/simple/src/App.tsx
@@ -39,7 +39,12 @@ const Clock = () => {
 
 const Timer = () => {
   return (
-    <TimerContainer>
+    <TimerContainer
+      onStart={(e) => console.log('start', e)}
+      onStop={(e) => console.log('stop', e)}
+      onComplete={(e) => console.log('complete', e)}
+      onReset={(e) => console.log('reset', e)}
+    >
       <TimerContext.Consumer>
         {(timer: TimerProps) => <MinimalTimer {...timer} />}
       </TimerContext.Consumer>
@@ -49,7 +54,11 @@ const Timer = () => {
 
 const Stopwatch = () => {
   return (
-    <StopwatchContainer>
+    <StopwatchContainer
+      onStart={(e) => console.log('start', e)}
+      onStop={(e) => console.log('stop', e)}
+      onReset={(e) => console.log('reset', e)}
+    >
       <StopwatchContext.Consumer>
         {(props: StopwatchProps) => <MinimalStopwatch {...props} />}
       </StopwatchContext.Consumer>
diff --git a/packages/stopwatch/src/container.tsx b/packages/stopwatch/src/container.tsx
index 2f008ac7..14064fb7 100644
--- a/packages/stopwatch/src/container.tsx
+++ b/packages/stopwatch/src/container.tsx
@@ -35,6 +35,12 @@ export const StopwatchContainer: React.FunctionComponent<
   StopwatchContainerProps
 > = (props): JSX.Element => {
   const { children } = props
+  const emptyFn = (_event: CustomEvent) => {
+    /* do nothing */
+  }
+  const onStart = props.onStart ?? emptyFn
+  const onStop = props.onStop ?? emptyFn
+  const onReset = props.onReset ?? emptyFn
   const [running, setRunning] = useState(false)
   const [elapsedTimeInLap, setElapsedTimeInLap] = useState(0)
   const [elapsedTimeTotal, setElapsedTimeTotal] = useState(0)
@@ -61,6 +67,7 @@ export const StopwatchContainer: React.FunctionComponent<
     if (running) {
       return
     }
+    onStart(new CustomEvent('start', {}))
     setStartTime(new Date())
     setRunning(true)
   }
@@ -69,6 +76,7 @@ export const StopwatchContainer: React.FunctionComponent<
     if (!running) {
       return
     }
+    onStop(new CustomEvent('stop', {}))
     updateElapsedTime()
     setRunning(false)
     moveLapToTotal()
@@ -86,6 +94,7 @@ export const StopwatchContainer: React.FunctionComponent<
     if (running) {
       stop()
     }
+    onReset(new CustomEvent('reset', {}))
     setElapsedTimeInLap(0)
     setElapsedTimeTotal(0)
   }
diff --git a/packages/stopwatch/src/types.ts b/packages/stopwatch/src/types.ts
index 68910ce2..76e30127 100644
--- a/packages/stopwatch/src/types.ts
+++ b/packages/stopwatch/src/types.ts
@@ -15,4 +15,7 @@ export type StopwatchProps = StopwatchValue & StopwatchActions
 export interface StopwatchContainerProps {
   refreshInterval?: number
   children: React.ReactElement | React.ReactElement[]
+  onStart?(event: CustomEvent): void
+  onStop?(event: CustomEvent): void
+  onReset?(event: CustomEvent): void
 }
diff --git a/packages/timer/src/container.tsx b/packages/timer/src/container.tsx
index f8c63539..9e4b8b4b 100644
--- a/packages/timer/src/container.tsx
+++ b/packages/timer/src/container.tsx
@@ -37,6 +37,13 @@ export const TimerContainer: React.FunctionComponent<TimerContainerProps> = (
   props
 ): JSX.Element => {
   const { children } = props
+  const emptyFn = (_event: CustomEvent) => {
+    /* do nothing */
+  }
+  const onStart = props.onStart ?? emptyFn
+  const onStop = props.onStop ?? emptyFn
+  const onComplete = props.onComplete ?? emptyFn
+  const onReset = props.onReset ?? emptyFn
   const [running, setRunning] = useState(false)
   const [targetDate, setTargetDate] = useState(new Date())
   const [remaining, setRemaining] = useState(0)
@@ -44,15 +51,21 @@ export const TimerContainer: React.FunctionComponent<TimerContainerProps> = (
   const refreshInterval = props.refreshInterval || 10 // default 10ms
 
   useInterval(() => {
+    tick()
+  }, refreshInterval)
+
+  function tick() {
+    // It can't be ticked if it is not running.
     if (running) {
       const remaining = calcRemaining(targetDate)
       setRemaining(remaining)
       if (remaining <= 0) {
         setRemaining(0)
         setRunning(false)
+        onComplete(new CustomEvent('complete', {}))
       }
     }
-  }, refreshInterval)
+  }
 
   function toggle() {
     if (running) {
@@ -63,19 +76,35 @@ export const TimerContainer: React.FunctionComponent<TimerContainerProps> = (
   }
 
   function reset() {
+    // First, stop the timer if it is running.
     if (running) {
       stop()
     }
     setRemaining(0)
+    onReset(new CustomEvent('reset', {}))
   }
 
   function start() {
+    // It can't be started if the remaining time is 0.
+    if (remaining <= 0) {
+      return
+    }
+    // It can't be started if it is already running.
+    if (running) {
+      return
+    }
     setTargetDate(new Date(Date.now() + remaining * 1000))
     setRunning(true)
+    onStart(new CustomEvent('start', {}))
   }
 
   function stop() {
+    // It can't be stopped if it is not running.
+    if (!running) {
+      return
+    }
     setRunning(false)
+    onStop(new CustomEvent('stop', {}))
   }
 
   return (
diff --git a/packages/timer/src/types.ts b/packages/timer/src/types.ts
index 8c990bca..88185c2d 100644
--- a/packages/timer/src/types.ts
+++ b/packages/timer/src/types.ts
@@ -17,4 +17,8 @@ export type TimerProps = TimerValue & TimerActions
 export interface TimerContainerProps {
   refreshInterval?: number
   children: React.ReactElement | React.ReactElement[]
+  onStart?(event: CustomEvent): void
+  onStop?(event: CustomEvent): void
+  onComplete?(event: CustomEvent): void
+  onReset?(event: CustomEvent): void
 }