diff --git a/.github/config/wordlist.txt b/.github/config/wordlist.txt
index ec05e6b..30d75d8 100644
--- a/.github/config/wordlist.txt
+++ b/.github/config/wordlist.txt
@@ -1,3 +1,4 @@
+abstractworkflowstep
 acyclic
 amstutz
 arrayschema
@@ -160,6 +161,9 @@ loadlistingrequirement
 localhost
 lookahead
 loopback
+loopinput
+loopoutputmethod
+loopworkflowstep
 maccallum
 macos
 mappredicate
@@ -194,6 +198,7 @@ outputbinding
 outputenumschema
 outputeval
 outputformat
+outputmethod
 outputschema
 outputsink
 outputsource
@@ -231,6 +236,7 @@ saladversion
 sbgenomics
 scatterfeaturerequirement
 scattermethod
+scatterworkflowstep
 schemadefrequirement
 schemaorg
 schemas
diff --git a/.github/workflows/cwltool.yml b/.github/workflows/cwltool.yml
index 9bb190a..f634623 100644
--- a/.github/workflows/cwltool.yml
+++ b/.github/workflows/cwltool.yml
@@ -24,7 +24,7 @@ jobs:
           python-version: '3.9.x'
 
       - name: Setup prerequirements
-        run: pip install "cwltest>=2.3" pytest-xdist "git+https://github.com/common-workflow-language/cwltool.git@main#egg=cwltool"
+        run: pip install "cwltest>=2.3" pytest-xdist "cwltool>=3.1.20240909164951"
 
       - name: Copy in cwltool-specific configuration for the cwltest pytest plugin
         run: cp "$(python -c 'from cwltool.tests.util import get_data; print(get_data("tests/cwl-conformance/cwltool-conftest.py"))')" conftest.py
@@ -33,4 +33,4 @@ jobs:
         run: cp conformance_tests.yaml conformance_tests.cwltest.yaml
 
       - name: Run tests against the reference runner
-        run: python -m pytest conformance_tests.cwltest.yaml -n auto -rs
+        run: python -m pytest conformance_tests.cwltest.yaml -n auto -rsfE
diff --git a/CONFORMANCE_TESTS.md b/CONFORMANCE_TESTS.md
index 2262870..8e401ac 100644
--- a/CONFORMANCE_TESTS.md
+++ b/CONFORMANCE_TESTS.md
@@ -144,7 +144,7 @@ We will use this single entry to explain the format
      Must include one or more of the following tags: `command_line_tool`, `expression_tool` or `workflow`.
      If the test does not test any optional features, the tag `required` is required.
 
-Because `conformance_tests.yaml` is a `schema-salad` processed document, [`$import`](https://www.commonwl.org/v1.2/SchemaSalad.html#Import)
+Because `conformance_tests.yaml` is a `schema-salad` processed document, [`$import`](https://www.commonwl.org/v1.3/SchemaSalad.html#Import)
 can be used to organize the tests into separate files.
 
 Currently, the main file is too big (over 3400 lines); we are slowly re-organizing it.
@@ -166,7 +166,7 @@ At any level, if there is an extra field, then that will be considered an error.
 An exception to this is `class: File` and `class: Directory` objects, the `cwl-runner` under test can add additional fields here without causing a test to fail.
 Likewise, if you don't want to test some aspect of a `class: File` or `class: Directory` object (like `nameext`) you can just omit it.
 
-[According to the CWL standards](https://www.commonwl.org/v1.2/CommandLineTool.html#File), the format of the `location` field in 
+[According to the CWL standards](https://www.commonwl.org/v1.3/CommandLineTool.html#File), the format of the `location` field in 
 `class: File` and `class: Directory` is implementation specific and we should not be testing them.
 Please remember to use `location: Any` for them.
 
diff --git a/Workflow.yml b/Workflow.yml
index 176de9a..6c20b8c 100644
--- a/Workflow.yml
+++ b/Workflow.yml
@@ -40,6 +40,12 @@ $graph:
       CWL group.
 
       ## Changelog
+      
+      * Added a new Loop feature to support iterative workflows. From a technical perspective, this led to the
+        introduction of a `LoopWorkflowStep`[#LoopWorkflowStep] element to represent iterative subworkflows, a
+        `LoopInput`[#LoopInput] object to handle termination conditions, and a `LoopOutputMethod`[#LoopOutputMethod]
+        feature to manage loop output data history.
+      
       See also the [CWL Command Line Tool Description, v1.3.0-dev1 changelog](CommandLineTool.html#Changelog).
       For other changes since CWL v1.0, see the
       [CWL Workflow Description, v1.1 changelog](https://www.commonwl.org/v1.1/Workflow.html#Changelog)
@@ -275,7 +281,7 @@ $graph:
 - type: record
   name: WorkflowStepInput
   extends: [Identified, InputSink, LoadContents, Labeled]
-  docParent: "#WorkflowStep"
+  docParent: "#AbstractWorkflowStep"
   doc: |
     The input of a workflow step connects an upstream parameter (from the
     workflow inputs, or the outputs of other workflows steps) with the input
@@ -301,7 +307,7 @@ $graph:
     in the workflow or workflow step requirements.
 
     If the sink parameter is an array, or named in a [workflow
-    scatter](#WorkflowStep) operation, there may be multiple inbound
+    scatter](#ScatterWorkflowStep) operation, there may be multiple inbound
     data links listed in the `source` field.  The values from the
     input links are merged depending on the method specified in the
     `linkMerge` field.  If both `linkMerge` and `pickValue` are null
@@ -338,7 +344,7 @@ $graph:
       3. Before `scatter` or `valueFrom`.
 
     This is specifically intended to be useful in combination with
-    [conditional execution](#WorkflowStep), where several upstream
+    [conditional execution](#AbstractWorkflowStep), where several upstream
     steps may be connected to a single input (`source` is a list), and
     skipped steps produce null values.
 
@@ -430,7 +436,7 @@ $graph:
 
 - type: record
   name: WorkflowStepOutput
-  docParent: "#WorkflowStep"
+  docParent: "#AbstractWorkflowStep"
   extends: Identified
   doc: |
     Associate an output parameter of the underlying process with a workflow
@@ -443,19 +449,68 @@ $graph:
     to connect the output value to downstream parameters.
 
 
+- name: LoopInput
+  type: record
+  extends: [Identified, OutputSink]
+  fields:
+    - name: default
+      type: ["null", File, Directory, Any]
+      doc: |
+        The default value for this parameter to use if either there is no
+        `outputSource` field, or the value produced by the `source` is `null`. The
+        default must be applied prior to scattering or evaluating `valueFrom`.
+      jsonldPredicate:
+        _id: "sld:default"
+        _container: "@list"
+        noLinkCheck: true
+    - name: valueFrom
+      type:
+        - "null"
+        - string
+        - Expression
+      jsonldPredicate: "cwl:valueFrom"
+      doc: |
+        To use valueFrom, [StepInputExpressionRequirement](#StepInputExpressionRequirement) must
+        be specified in the workflow or workflow step requirements.
+
+        If `valueFrom` is a constant string value, use this as the value for
+        this input parameter.
+
+        If `valueFrom` is a parameter reference or expression, it must be
+        evaluated to yield the actual value to be assigned to the input field.
+
+        The `self` value in the parameter reference or expression must be
+        `null` if there is no `source` field, or the value of the
+        parameter(s) specified in the `source` field.
+
+        The value of `inputs` in the parameter reference or expression must be
+        the input object to the previous iteration of the workflow step (or the initial
+        inputs for the first iteration).
+
+
 - name: ScatterMethod
   type: enum
-  docParent: "#WorkflowStep"
-  doc: The scatter method, as described in [workflow step scatter](#WorkflowStep).
+  docParent: "#ScatterWorkflowStep"
+  doc: The scatter method, as described in [workflow step scatter](#ScatterWorkflowStep).
   symbols:
     - dotproduct
     - nested_crossproduct
     - flat_crossproduct
 
 
-- name: WorkflowStep
+- name: LoopOutputMethod
+  type: enum
+  docParent: "#LoopWorkflowStep"
+  doc: The loop output method, as described in [workflow step loop](#LoopWorkflowStep).
+  symbols:
+    - last_iteration
+    - all_iterations
+
+
+- name: AbstractWorkflowStep
   type: record
   extends: [Identified, Labeled, sld:Documented]
+  abstract: true
   docParent: "#Workflow"
   doc: |
     A workflow step is an executable element of a workflow.  It specifies the
@@ -463,71 +518,23 @@ $graph:
     `Workflow`) in the `run` field and connects the input and output parameters
     of the underlying process to workflow parameters.
 
-    # Scatter/gather
-
-    To use scatter/gather,
-    [ScatterFeatureRequirement](#ScatterFeatureRequirement) must be specified
-    in the workflow or workflow step requirements.
-
-    A "scatter" operation specifies that the associated workflow step or
-    subworkflow should execute separately over a list of input elements.  Each
-    job making up a scatter operation is independent and may be executed
-    concurrently.
-
-    The `scatter` field specifies one or more input parameters which will be
-    scattered.  An input parameter may be listed more than once.  The declared
-    type of each input parameter implicitly becomes an array of items of the
-    input parameter type.  If a parameter is listed more than once, it becomes
-    a nested array.  As a result, upstream parameters which are connected to
-    scattered parameters must be arrays.
-
-    All output parameter types are also implicitly wrapped in arrays.  Each job
-    in the scatter results in an entry in the output array.
-
-    If any scattered parameter runtime value is an empty array, all outputs are
-    set to empty arrays and no work is done for the step, according to
-    applicable scattering rules.
-
-    If `scatter` declares more than one input parameter, `scatterMethod`
-    describes how to decompose the input into a discrete set of jobs.
-
-      * **dotproduct** specifies that each of the input arrays are aligned and one
-          element taken from each array to construct each job.  It is an error
-          if all input arrays are not the same length.
-
-      * **nested_crossproduct** specifies the Cartesian product of the inputs,
-          producing a job for every combination of the scattered inputs.  The
-          output must be nested arrays for each level of scattering, in the
-          order that the input arrays are listed in the `scatter` field.
-
-      * **flat_crossproduct** specifies the Cartesian product of the inputs,
-          producing a job for every combination of the scattered inputs.  The
-          output arrays must be flattened to a single level, but otherwise listed in the
-          order that the input arrays are listed in the `scatter` field.
-
     # Conditional execution (Optional)
 
     Conditional execution makes execution of a step conditional on an
     expression.  A step that is not executed is "skipped".  A skipped
     step produces `null` for all output parameters.
 
-    The condition is evaluated after `scatter`, using the input object
-    of each individual scatter job.  This means over a set of scatter
-    jobs, some may be executed and some may be skipped.  When the
-    results are gathered, skipped steps must be `null` in the output
-    arrays.
-
     The `when` field controls conditional execution.  This is an
     expression that must be evaluated with `inputs` bound to the step
     input object (or individual scatter job), and returns a boolean
     value.  It is an error if this expression returns a value other
     than `true` or `false`.
 
-    Conditionals in CWL are an optional feature and are not required
-    to be implemented by all consumers of CWL documents.  An
-    implementation that does not support conditionals must return a
-    fatal error when attempting to execute a workflow that uses
-    conditional constructs the implementation does not support.
+    Conditional execution in CWL is an optional feature and is not required
+    to be implemented by all consumers of CWL documents. An implementation that
+    does not support conditional executions must return a fatal error when
+    attempting to execute a workflow that uses conditional constructs the
+    implementation does not support.
 
     # Subworkflows
 
@@ -603,6 +610,68 @@ $graph:
         If defined, only run the step when the expression evaluates to
         `true`.  If `false` the step is skipped.  A skipped step
         produces a `null` on each output.
+
+
+- name: WorkflowStep
+  type: record
+  extends: AbstractWorkflowStep
+  docParent: "#Workflow"
+
+
+- name: ScatterWorkflowStep
+  type: record
+  extends: AbstractWorkflowStep
+  docParent: "#Workflow"
+  doc: |
+    To use scatter/gather,
+    [ScatterFeatureRequirement](#ScatterFeatureRequirement) must be specified
+    in the workflow or workflow step requirements.
+
+    A "scatter" operation specifies that the associated workflow step or
+    subworkflow should execute separately over a list of input elements.  Each
+    job making up a scatter operation is independent and may be executed
+    concurrently.
+
+    The `scatter` field specifies one or more input parameters which will be
+    scattered.  An input parameter may be listed more than once.  The declared
+    type of each input parameter implicitly becomes an array of items of the
+    input parameter type.  If a parameter is listed more than once, it becomes
+    a nested array.  As a result, upstream parameters which are connected to
+    scattered parameters must be arrays.
+
+    All output parameter types are also implicitly wrapped in arrays.  Each job
+    in the scatter results in an entry in the output array.
+
+    If any scattered parameter runtime value is an empty array, all outputs are
+    set to empty arrays and no work is done for the step, according to
+    applicable scattering rules.
+
+    If `scatter` declares more than one input parameter, `scatterMethod`
+    describes how to decompose the input into a discrete set of jobs.
+
+      * **dotproduct** specifies that each of the input arrays are aligned and one
+          element taken from each array to construct each job.  It is an error
+          if all input arrays are not the same length.
+
+      * **nested_crossproduct** specifies the Cartesian product of the inputs,
+          producing a job for every combination of the scattered inputs.  The
+          output must be nested arrays for each level of scattering, in the
+          order that the input arrays are listed in the `scatter` field.
+
+      * **flat_crossproduct** specifies the Cartesian product of the inputs,
+          producing a job for every combination of the scattered inputs.  The
+          output arrays must be flattened to a single level, but otherwise listed in the
+          order that the input arrays are listed in the `scatter` field.
+
+    # Conditional execution (Optional)
+
+    The condition is evaluated after `scatter`, using the input object
+    of each individual scatter job.  This means over a set of scatter
+    jobs, some may be executed and some may be skipped.  When the
+    results are gathered, skipped steps must be `null` in the output
+    arrays.
+
+  fields:
     - name: scatter
       type:
         - string?
@@ -621,6 +690,74 @@ $graph:
         "_type": "@vocab"
 
 
+- name: LoopWorkflowStep
+  type: record
+  extends: AbstractWorkflowStep
+  docParent: "#Workflow"
+  doc: |
+    # Iterative execution (Optional)
+
+    The `loop` field controls iterative execution. It defines the input
+    parameters of the loop iterations after the first one (inputs of the
+    first iteration are the step input parameters, as usual). If no
+    `loop` rule is specified for a given step `in` field, the initial
+    value is kept constant among all iterations.
+
+    When a `loop` field is present, the `when` field is mandatory. It is
+    evaluated before each loop iteration and acts as a termination condition:
+    as soon as the `when` expression evaluates to `false`, the loop terminates
+    and the step outputs are propagated to the subsequent workflow steps.
+
+    The `outputMethod` field describes how to deal with loop outputs after
+    termination:
+
+      * **last_iteration** specifies that only the last computed element for
+        each output parameter should be propagated to the subsequent steps.
+        This is the default value.
+
+      * **all_iterations** specifies that an array with all output values
+        computed at the end of each loop iteration should be propagated to
+        the subsequent steps. Elements in the array must be ordered according
+        to the loop iterations that produced them.
+
+    Iterative execution in CWL is an optional feature and is not required
+    to be implemented by all consumers of CWL documents. An implementation that
+    does not support iterative executions must return a fatal error when
+    attempting to execute a workflow that uses iterative constructs the
+    implementation does not support.
+
+  fields:
+    - name: loop
+      doc: |
+        Defines the input parameters of the loop iterations after the first one
+        (inputs of the first iteration are the step input parameters). If no
+        `loop` rule is specified for a given step `in` field, the initial value
+        is kept constant among all iterations.
+      type: LoopInput[]?
+      jsonldPredicate:
+        _id: "cwl:loop"
+        mapSubject: id
+        mapPredicate: outputSource
+    - name: outputMethod
+      doc: |
+        If not specified, the default method is "last_iteration".
+      type: LoopOutputMethod?
+      default: last_iteration
+      jsonldPredicate:
+        "_id": "cwl:outputMethod"
+        "_type": "@vocab"
+    - name: when
+      type:
+        - Expression
+      jsonldPredicate: "cwl:when"
+      doc: |
+        Only run the next iteration when the expression evaluates to `true`.
+        If the first iteration evaluates to `false` the step is skipped.
+        A skipped step produces a `null` on each output if the `outputMethod`
+        is set to `last_iteration`, and an empty array if the `outputMethod`
+        is set to `all_iterations`.
+
+
 - name: Workflow
   type: record
   extends: "#Process"
@@ -701,7 +838,7 @@ $graph:
         concurrently, provided that dependencies between steps are met.
       type:
         - type: array
-          items: "#WorkflowStep"
+          items: AbstractWorkflowStep
       jsonldPredicate:
           mapSubject: id
 
@@ -711,7 +848,7 @@ $graph:
   extends: ProcessRequirement
   doc: |
     Indicates that the workflow platform must support nested workflows in
-    the `run` field of [WorkflowStep](#WorkflowStep).
+    the `run` field of [AbstractWorkflowStep](#AbstractWorkflowStep).
   fields:
     - name: "class"
       type:
@@ -729,7 +866,7 @@ $graph:
   extends: ProcessRequirement
   doc: |
     Indicates that the workflow platform must support the `scatter` and
-    `scatterMethod` fields of [WorkflowStep](#WorkflowStep).
+    `scatterMethod` fields of [ScatterWorkflowStep](#ScatterWorkflowStep).
   fields:
     - name: "class"
       type:
diff --git a/conformance_tests.yaml b/conformance_tests.yaml
index 8d397d7..ee8c0a6 100644
--- a/conformance_tests.yaml
+++ b/conformance_tests.yaml
@@ -3525,3 +3525,5 @@
       an_array_of_mixed_booleans: [ false, true, false ]
       an_array_of_trues: [ true, true, true ]
       an_int: 42
+
+- $import: tests/loop/test-index.yaml
diff --git a/design-documents/conditionals-2019.md b/design-documents/conditionals-2019.md
index 34f7d69..ca581c3 100644
--- a/design-documents/conditionals-2019.md
+++ b/design-documents/conditionals-2019.md
@@ -10,7 +10,7 @@ This is a documentation of the design and design decisions for conditionals as o
 ![dual scatter nested](conditionals/conditional-patterns-3.png)
 ![dual scatter flattened](conditionals/conditional-patterns-4.png)
 
-The design adds a new field `when` to a `WorkflowStep`. This field is an expression that
+The design adds a new field `when` to a `AbstractWorkflowStep`. This field is an expression that
 evaluates to `True` or `False`. The executor runs the step if the value is `True`,
 skips it if `False`. A skipped step produces `null` values on all its outputs.
 
@@ -114,7 +114,7 @@ outputs:
     pickValue: first_non_null
 ```
 
-The new syntax adds a single field to `WorkflowStep` (`when`) and a new 
+The new syntax adds a single field to `AbstractWorkflowStep` (`when`) and a new 
 operator called `pickValue` to the `WorkflowStepInput` and 
 `WorkflowOutputParameter`. This is a fairly non-intrusive 
 modification, fully backwards compatible (it's an addition, not a modification) and allows
diff --git a/requirements.txt b/requirements.txt
index f5c7cc1..5d99a3a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1 @@
-cwltool
+cwltool
\ No newline at end of file
diff --git a/tests/conditionals/test-index.yaml b/tests/conditionals/test-index.yaml
index cee57e9..f646a56 100644
--- a/tests/conditionals/test-index.yaml
+++ b/tests/conditionals/test-index.yaml
@@ -20,7 +20,7 @@
   tags: [ conditional, inline_javascript, workflow ]
 
 - id: direct_required
-  doc: Conditional using intermediate WorkflowStep.in input
+  doc: Conditional using intermediate AbstractWorkflowStep.in input
   tool: cond-wf-002.cwl
   job: val.1.job.yaml
   output:
@@ -186,7 +186,7 @@
     out1: [
       "foo 2", "foo 4", "foo 6",
       "bar 1", "bar 3", "bar 5"]
-  tags: [ conditional, inline_javascript, scatter, multiple, workflow ]
+  tags: [ conditional, inline_javascript, scatter, multiple_input, workflow ]
 
 - id: direct_optional_null_result_nojs
   doc: simplest conditional pattern (true), no javascript
@@ -205,7 +205,7 @@
   tags: [ conditional, workflow ]
 
 - id: direct_required_nojs
-  doc: Conditional using intermediate WorkflowStep.in input; no javascript
+  doc: Conditional using intermediate AbstractWorkflowStep.in input; no javascript
   tool: cond-wf-002_nojs.cwl
   job: val.1.job.yaml
   output:
@@ -371,7 +371,7 @@
     out1: [
       "foo 2", "foo 4", "foo 6",
       "bar 1", "bar 3", "bar 5"]
-  tags: [ conditional, scatter, multiple, workflow ]
+  tags: [ conditional, scatter, multiple_input, workflow ]
 
 
 - id: cond-with-defaults-1
@@ -395,7 +395,7 @@
             "size": 34
         }
     ]
-  tags: [ conditional, scatter, multiple, workflow ]
+  tags: [ conditional, scatter, multiple_input, workflow ]
 
 - id: cond-with-defaults-2
   doc: "Default inputs, choose step to run based on what was provided, second case"
@@ -411,4 +411,4 @@
             "size": 12
         }
     ]
-  tags: [ conditional, scatter, multiple, workflow ]
+  tags: [ conditional, scatter, multiple_input, workflow ]
diff --git a/tests/loop/all-output-loop-no-iteration.cwl b/tests/loop/all-output-loop-no-iteration.cwl
new file mode 100644
index 0000000..97d6240
--- /dev/null
+++ b/tests/loop/all-output-loop-no-iteration.cwl
@@ -0,0 +1,28 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 1)
+    loop:
+      i1: o1
+    outputMethod: all_iterations
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + 1};}
+    in:
+      i1: i1
+    out: [o1]
diff --git a/tests/loop/all-output-loop.cwl b/tests/loop/all-output-loop.cwl
new file mode 100644
index 0000000..7001731
--- /dev/null
+++ b/tests/loop/all-output-loop.cwl
@@ -0,0 +1,28 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1: o1
+    outputMethod: all_iterations
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + 1};}
+    in:
+      i1: i1
+    out: [o1]
diff --git a/tests/loop/default-value-loop.cwl b/tests/loop/default-value-loop.cwl
new file mode 100644
index 0000000..fb5e00e
--- /dev/null
+++ b/tests/loop/default-value-loop.cwl
@@ -0,0 +1,47 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  ScatterFeatureRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: loop/o1
+    pickValue: all_non_null
+steps:
+  loop:
+    when: $(inputs.i1 < 20)
+    loop:
+      i1:
+        outputSource: o1
+        default: 5
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+      outputs:
+        o1:
+          type: int?
+          outputSource: big_values/o1
+      steps:
+        big_values:
+          when: $(inputs.i1 >= 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 3};}
+          in:
+            i1: i1
+          out: [ o1 ]
+    in:
+      i1: i1
+    out: [ o1 ]
diff --git a/tests/loop/invalid-multi-source-loop-no-requirement.cwl b/tests/loop/invalid-multi-source-loop-no-requirement.cwl
new file mode 100644
index 0000000..aa83257
--- /dev/null
+++ b/tests/loop/invalid-multi-source-loop-no-requirement.cwl
@@ -0,0 +1,64 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  ScatterFeatureRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: [loop/osmall, loop/obig]
+    linkMerge: merge_flattened
+    pickValue: all_non_null
+steps:
+  loop:
+    when: $(inputs.i1 < 20)
+    loop:
+      i1:
+        outputSource: [ osmall, obig ]
+        pickValue: the_only_non_null
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+      outputs:
+        osmall:
+          type: int?
+          outputSource: small_values/o1
+        obig:
+          type: int?
+          outputSource: big_values/o1
+      steps:
+        small_values:
+          when: $(inputs.i1 < 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 1};}
+          in:
+            i1: i1
+          out: [o1]
+        big_values:
+          when: $(inputs.i1 >= 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 3};}
+          in:
+            i1: i1
+          out: [ o1 ]
+    in:
+      i1: i1
+    out: [osmall, obig]
diff --git a/tests/loop/invalid-value-from-loop-no-requirement.cwl b/tests/loop/invalid-value-from-loop-no-requirement.cwl
new file mode 100644
index 0000000..52edf39
--- /dev/null
+++ b/tests/loop/invalid-value-from-loop-no-requirement.cwl
@@ -0,0 +1,32 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1:
+        valueFrom: $(inputs.i1 + 1)
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + inputs.i2};}
+    in:
+      i1: i1
+      i2: i2
+    out: [o1]
diff --git a/tests/loop/loop-inside-loop-all.cwl b/tests/loop/loop-inside-loop-all.cwl
new file mode 100644
index 0000000..3c2da4b
--- /dev/null
+++ b/tests/loop/loop-inside-loop-all.cwl
@@ -0,0 +1,58 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  ScatterFeatureRequirement: {}
+  StepInputExpressionRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type:
+      type: array
+      items:
+        type: array
+        items: int
+    outputSource: loop1/o1
+steps:
+  loop1:
+    when: $(inputs.i2 < 4)
+    loop:
+      i2:
+        valueFrom: $(inputs.i2 + 1)
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1:
+          type: int[]
+          outputSource: loop2/o1
+      steps:
+        loop2:
+          when: $(inputs.i1 <= inputs.i2)
+          loop:
+            i1: o1
+          outputMethod: all_iterations
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+              i2: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 1};}
+          in:
+            i1: i1
+            i2: i2
+          out: [o1]
+    in:
+      i1: i1
+      i2: i2
+    out: [o1]
diff --git a/tests/loop/loop-inside-loop.cwl b/tests/loop/loop-inside-loop.cwl
new file mode 100644
index 0000000..2681eb5
--- /dev/null
+++ b/tests/loop/loop-inside-loop.cwl
@@ -0,0 +1,54 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  ScatterFeatureRequirement: {}
+  StepInputExpressionRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: loop1/o1
+steps:
+  loop1:
+    when: $(inputs.i2 < 4)
+    loop:
+      i2:
+        valueFrom: $(inputs.i2 + 1)
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1:
+          type: int
+          outputSource: loop2/o1
+      steps:
+        loop2:
+          when: $(inputs.i1 <= inputs.i2)
+          loop:
+            i1: o1
+          outputMethod: last_iteration
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+              i2: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 1};}
+          in:
+            i1: i1
+            i2: i2
+          out: [o1]
+    in:
+      i1: i1
+      i2: i2
+    out: [o1]
diff --git a/tests/loop/loop-inside-scatter-job.yml b/tests/loop/loop-inside-scatter-job.yml
new file mode 100644
index 0000000..0d95d06
--- /dev/null
+++ b/tests/loop/loop-inside-scatter-job.yml
@@ -0,0 +1,2 @@
+i1: [1, 2, 3, 4, 5]
+i2: 1
\ No newline at end of file
diff --git a/tests/loop/loop-inside-scatter.cwl b/tests/loop/loop-inside-scatter.cwl
new file mode 100644
index 0000000..7c6db45
--- /dev/null
+++ b/tests/loop/loop-inside-scatter.cwl
@@ -0,0 +1,49 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  ScatterFeatureRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int[]
+  i2: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: scatter/o1
+steps:
+  scatter:
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1:
+          type: int
+          outputSource: subworkflow/o1
+      steps:
+        subworkflow:
+          when: $(inputs.i1 < 10)
+          loop:
+            i1: o1
+          outputMethod: last_iteration
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+              i2: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + inputs.i2};}
+          in:
+            i1: i1
+            i2: i2
+          out: [o1]
+    in:
+      i1: i1
+      i2: i2
+    scatter: i1
+    out: [o1]
diff --git a/tests/loop/multi-source-loop-no-pick-value.cwl b/tests/loop/multi-source-loop-no-pick-value.cwl
new file mode 100644
index 0000000..7587681
--- /dev/null
+++ b/tests/loop/multi-source-loop-no-pick-value.cwl
@@ -0,0 +1,64 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  MultipleInputFeatureRequirement: {}
+  ScatterFeatureRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: [loop/osmall, loop/obig]
+    linkMerge: merge_flattened
+steps:
+  loop:
+    when: $(inputs.i1 < 5)
+    loop:
+      i1:
+        outputSource: [ osmall, obig ]
+        valueFrom: $(self[0])
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+      outputs:
+        osmall:
+          type: int?
+          outputSource: small_values/o1
+        obig:
+          type: int?
+          outputSource: big_values/o1
+      steps:
+        small_values:
+          when: $(inputs.i1 < 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 1};}
+          in:
+            i1: i1
+          out: [o1]
+        big_values:
+          when: $(inputs.i1 >= 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 3};}
+          in:
+            i1: i1
+          out: [ o1 ]
+    in:
+      i1: i1
+    out: [osmall, obig]
diff --git a/tests/loop/multi-source-loop.cwl b/tests/loop/multi-source-loop.cwl
new file mode 100644
index 0000000..af0adc8
--- /dev/null
+++ b/tests/loop/multi-source-loop.cwl
@@ -0,0 +1,65 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  MultipleInputFeatureRequirement: {}
+  ScatterFeatureRequirement: {}
+  SubworkflowFeatureRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int[]
+    outputSource: [loop/osmall, loop/obig]
+    linkMerge: merge_flattened
+    pickValue: all_non_null
+steps:
+  loop:
+    when: $(inputs.i1 < 20)
+    loop:
+      i1:
+        outputSource: [ osmall, obig ]
+        pickValue: the_only_non_null
+    outputMethod: all_iterations
+    run:
+      class: Workflow
+      inputs:
+        i1: int
+      outputs:
+        osmall:
+          type: int?
+          outputSource: small_values/o1
+        obig:
+          type: int?
+          outputSource: big_values/o1
+      steps:
+        small_values:
+          when: $(inputs.i1 < 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 1};}
+          in:
+            i1: i1
+          out: [o1]
+        big_values:
+          when: $(inputs.i1 >= 5)
+          run:
+            class: ExpressionTool
+            inputs:
+              i1: int
+            outputs:
+              o1: int
+            expression: >
+              ${return {'o1': inputs.i1 + 3};}
+          in:
+            i1: i1
+          out: [ o1 ]
+    in:
+      i1: i1
+    out: [osmall, obig]
diff --git a/tests/loop/single-var-loop-job.yml b/tests/loop/single-var-loop-job.yml
new file mode 100644
index 0000000..d491f14
--- /dev/null
+++ b/tests/loop/single-var-loop-job.yml
@@ -0,0 +1 @@
+i1: 1
\ No newline at end of file
diff --git a/tests/loop/single-var-loop-no-iteration.cwl b/tests/loop/single-var-loop-no-iteration.cwl
new file mode 100644
index 0000000..9dd60d7
--- /dev/null
+++ b/tests/loop/single-var-loop-no-iteration.cwl
@@ -0,0 +1,28 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 1)
+    loop:
+      i1: o1
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + 1};}
+    in:
+      i1: i1
+    out: [o1]
diff --git a/tests/loop/single-var-loop.cwl b/tests/loop/single-var-loop.cwl
new file mode 100644
index 0000000..e94eb05
--- /dev/null
+++ b/tests/loop/single-var-loop.cwl
@@ -0,0 +1,28 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1: o1
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + 1};}
+    in:
+      i1: i1
+    out: [o1]
diff --git a/tests/loop/test-index.yaml b/tests/loop/test-index.yaml
new file mode 100644
index 0000000..1122f11
--- /dev/null
+++ b/tests/loop/test-index.yaml
@@ -0,0 +1,109 @@
+- job: single-var-loop-job.yml
+  tool: single-var-loop.cwl
+  id: loop_single_variable
+  doc: "Test a simple loop case with a single variable"
+  output:
+    o1: 10
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: single-var-loop-no-iteration.cwl
+  id: loop_single_variable_no_iteration
+  doc: "Test a simple loop case with a single variable and a false condition"
+  output:
+    o1: null
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: two-vars-loop.cwl
+  id: loop_two_variables
+  doc: "Test a loop case with two variables, which are both back-propagated between iterations"
+  output:
+    o1: 10
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: two-vars-loop-2.cwl
+  id: loop_two_variables_single_backpropagation
+  doc: "Test a loop case with two variables, but when only one of them is back-propagated between iterations"
+  output:
+    o1: 10
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: all-output-loop.cwl
+  id: loop_with_all_output_method
+  doc: "Test a loop case with outputMethod set to all_iterations"
+  output:
+    o1: [2, 3, 4, 5, 6, 7, 8, 9, 10]
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: all-output-loop-no-iteration.cwl
+  id: loop_with_all_output_method_no_iteration
+  doc: "Test a loop case with outputMethod set to all_iterations and a false condition"
+  output:
+    o1: []
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: value-from-loop.cwl
+  id: loop_value_from
+  doc: "Test a loop case with a variable generated by a valueFrom directive"
+  output:
+    o1: 10
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: invalid-value-from-loop-no-requirement.cwl
+  id: loop_value_from_fail_no_requirement
+  doc: "Test that a workflow loop fails if a valueFrom directive is specified without StepInputExpressionRequirement"
+  should_fail: true
+  tags: [ conditional, loop, inline_javascript, workflow ]
+
+- job: loop-inside-scatter-job.yml
+  tool: loop-inside-scatter.cwl
+  id: loop_inside_scatter
+  doc: "Test a loop subworkflow inside a scatter step"
+  output:
+    o1: [10, 10, 10, 10, 10]
+  tags: [ conditional, loop, inline_javascript, scatter, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: loop-inside-loop.cwl
+  id: loop_nested
+  doc: "Test a workflow with two nested loops"
+  output:
+    o1: [2, 3, 4]
+  tags: [ conditional, loop, inline_javascript, scatter, workflow ]
+
+- job: two-vars-loop-job.yml
+  tool: loop-inside-loop-all.cwl
+  id: loop_nested_all
+  doc: "Test a workflow with two nested loops, both with outputMethod set to all_iterations"
+  output:
+    o1: [[2], [2, 3], [2, 3, 4]]
+  tags: [ conditional, loop, inline_javascript, scatter, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: multi-source-loop.cwl
+  id: loop_multi_source_input
+  doc: "Test a loop with two sources, which are selected through a pickValue directive"
+  output:
+    o1: [2, 3, 4, 5, 8, 11, 14, 17, 20]
+  tags: [ conditional, loop, inline_javascript, multiple_input, scatter, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: invalid-multi-source-loop-no-requirement.cwl
+  id: loop_multi_source_input_fail_no_requirement
+  doc: "Test that a loop with two sources fails without MultipleInputFeatureRequirement"
+  should_fail: true
+  tags: [ conditional, loop, inline_javascript, multiple_input, scatter, workflow ]
+
+- job: single-var-loop-job.yml
+  tool: default-value-loop.cwl
+  id: loop_defaultvalue
+  output:
+    o1: [8, 11, 14, 17, 20]
+  doc: "Test a loop whose source has a default value"
+  tags: [ conditional, loop, inline_javascript, scatter, workflow ]
diff --git a/tests/loop/two-vars-loop-2.cwl b/tests/loop/two-vars-loop-2.cwl
new file mode 100644
index 0000000..decae00
--- /dev/null
+++ b/tests/loop/two-vars-loop-2.cwl
@@ -0,0 +1,31 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1: o1
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + inputs.i2};}
+    in:
+      i1: i1
+      i2: i2
+    out: [o1]
diff --git a/tests/loop/two-vars-loop-job.yml b/tests/loop/two-vars-loop-job.yml
new file mode 100644
index 0000000..012ed44
--- /dev/null
+++ b/tests/loop/two-vars-loop-job.yml
@@ -0,0 +1,2 @@
+i1: 1
+i2: 1
\ No newline at end of file
diff --git a/tests/loop/two-vars-loop.cwl b/tests/loop/two-vars-loop.cwl
new file mode 100644
index 0000000..ccb7e4a
--- /dev/null
+++ b/tests/loop/two-vars-loop.cwl
@@ -0,0 +1,33 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1: o1
+      i2: o2
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1: int
+        o2: int
+      expression: >
+        ${return {'o1': inputs.i1 + inputs.i2, 'o2': inputs.i2};}
+    in:
+      i1: i1
+      i2: i2
+    out: [o1, o2]
diff --git a/tests/loop/value-from-loop.cwl b/tests/loop/value-from-loop.cwl
new file mode 100644
index 0000000..835af1b
--- /dev/null
+++ b/tests/loop/value-from-loop.cwl
@@ -0,0 +1,33 @@
+#!/usr/bin/env cwl-runner
+cwlVersion: v1.3.0-dev1
+class: Workflow
+requirements:
+  InlineJavascriptRequirement: {}
+  StepInputExpressionRequirement: {}
+inputs:
+  i1: int
+  i2: int
+outputs:
+  o1:
+    type: int
+    outputSource: subworkflow/o1
+steps:
+  subworkflow:
+    when: $(inputs.i1 < 10)
+    loop:
+      i1:
+        valueFrom: $(inputs.i1 + 1)
+    outputMethod: last_iteration
+    run:
+      class: ExpressionTool
+      inputs:
+        i1: int
+        i2: int
+      outputs:
+        o1: int
+      expression: >
+        ${return {'o1': inputs.i1 + inputs.i2};}
+    in:
+      i1: i1
+      i2: i2
+    out: [o1]