diff --git a/tests/test_test_api_adapter.py b/tests/test_test_api_adapter.py
index 9f23559..2af21d1 100644
--- a/tests/test_test_api_adapter.py
+++ b/tests/test_test_api_adapter.py
@@ -95,6 +95,27 @@ def test_get_running_workflow():
     assert response["variables"] == {"x": 1}
 
 
+def test_get_running_steps_when_none_running():
+    # Arrange
+    utaa = UnitTestWorkflowAPIAdapter()
+    response = utaa.create_workflow(workflow_definition={"name": "blah"})
+    wfid = response["id"]
+    response = utaa.create_running_workflow(
+        user_id="dlister",
+        workflow_id=wfid,
+        project_id=TEST_PROJECT_ID,
+        variables={"x": 1},
+    )
+    rwfid = response["id"]
+
+    # Act
+    response, _ = utaa.get_running_steps(running_workflow_id=rwfid)
+
+    # Assert
+    assert response["count"] == 0
+    assert response["steps"] == []
+
+
 def test_set_running_workflow_done_when_success():
     # Arrange
     utaa = UnitTestWorkflowAPIAdapter()
diff --git a/tests/wapi_adapter.py b/tests/wapi_adapter.py
index 4fa260a..aaa633d 100644
--- a/tests/wapi_adapter.py
+++ b/tests/wapi_adapter.py
@@ -102,6 +102,12 @@ def get_running_workflow(
         response["id"] = running_workflow_id
         return response, 0
 
+    def get_running_steps(
+        self, *, running_workflow_id: str
+    ) -> tuple[dict[str, Any], int]:
+        # Does nothing at the moment - this is used for the STOP logic.
+        return {"count": 0, "steps": []}, 0
+
     def set_running_workflow_done(
         self,
         *,
diff --git a/workflow/workflow_abc.py b/workflow/workflow_abc.py
index d276219..1d31d93 100644
--- a/workflow/workflow_abc.py
+++ b/workflow/workflow_abc.py
@@ -149,6 +149,22 @@ def get_running_workflow(
         # }
         # If not present an empty dictionary should be returned.
 
+    @abstractmethod
+    def get_running_steps(
+        self, *, running_workflow_id: str
+    ) -> tuple[dict[str, Any], int]:
+        """Get a list of steps (their names) that are currently running for the
+        given RunningWorkflow Record"""
+        # Should return:
+        # {
+        #    "count": 1,
+        #    "steps": [
+        #       {
+        #           "name:": "step-1234"
+        #       }
+        #    ]
+        # }
+
     @abstractmethod
     def set_running_workflow_done(
         self,
@@ -201,7 +217,7 @@ def get_running_workflow_step(
         # can be expected in the response: -
         #
         #       "prior_running_workflow_step": {
-        #          "id": "r-worflkow-step-00000000-0000-0000-0000-000000000001",
+        #          "id": "r-workflow-step-00000000-0000-0000-0000-000000000001",
         #       },
 
     @abstractmethod
diff --git a/workflow/workflow_engine.py b/workflow/workflow_engine.py
index 3be7a42..15b409a 100644
--- a/workflow/workflow_engine.py
+++ b/workflow/workflow_engine.py
@@ -77,7 +77,7 @@ def handle_message(self, msg: Message) -> None:
 
     def _handle_workflow_message(self, msg: WorkflowMessage) -> None:
         """WorkflowMessages signal the need to start (or stop) a workflow using its
-        'action' string field (one of 'START' or 'START').
+        'action' string field (one of 'START' or 'STOP').
         The message contains a 'running_workflow' field that contains the UUID
         of an existing RunningWorkflow record in the DM. Using this
         we can locate the Workflow record and interrogate that to identify which
@@ -85,17 +85,15 @@ def _handle_workflow_message(self, msg: WorkflowMessage) -> None:
         assert msg
 
         _LOGGER.info("WorkflowMessage:\n%s", str(msg))
-        assert msg.action in ["START", "STOP"]
+        if msg.action not in ["START", "STOP"]:
+            _LOGGER.error("Ignoring unsupported action (%s)", msg.action)
+            return
 
         r_wfid = msg.running_workflow
         if msg.action == "START":
             self._handle_workflow_start_message(r_wfid)
         else:
-            # STOP is not implemented yet and probably not for some time.
-            # So just log and ignore for now!
-            _LOGGER.warning(
-                "Got STOP action for %s - but it's not implemented yet!", r_wfid
-            )
+            self._handle_workflow_stop_message(r_wfid)
 
     def _handle_workflow_start_message(self, r_wfid: str) -> None:
         """Logic to handle a START message. This is the beginning of a new
@@ -142,6 +140,40 @@ def _handle_workflow_start_message(self, r_wfid: str) -> None:
         # and error, stopping it. There will be no Pod event as the launch has failed.
         self._launch(rwf=rwf_response, rwfs_id=r_wfsid, step=first_step)
 
+    def _handle_workflow_stop_message(self, r_wfid: str) -> None:
+        """Logic to handle a STOP message."""
+        # Do nothing if the running workflow has already stopped.
+        rwf_response, _ = self._wapi_adapter.get_running_workflow(
+            running_workflow_id=r_wfid
+        )
+        _LOGGER.debug(
+            "API.get_running_workflow(%s) returned: -\n%s", r_wfid, str(rwf_response)
+        )
+        if not rwf_response:
+            _LOGGER.debug("Running workflow does not exist (%s)", r_wfid)
+            return
+        elif rwf_response["done"] is True:
+            _LOGGER.debug("Running workflow already stopped (%s)", r_wfid)
+            return
+
+        # For this version all we can do is check that no steps are running.
+        # If no steps are running we can safely mark the running workflow as stopped.
+        response, _ = self._wapi_adapter.get_running_steps(running_workflow_id=r_wfid)
+        _LOGGER.debug(
+            "API.get_running_steps(%s) returned: -\n%s", r_wfid, str(response)
+        )
+        if response:
+            if count := response["count"]:
+                msg: str = "1 step is" if count == 1 else f"{count} steps are"
+                _LOGGER.debug("Ignoring STOP for %s. %s still running", r_wfid, msg)
+            else:
+                self._wapi_adapter.set_running_workflow_done(
+                    running_workflow_id=r_wfid,
+                    success=False,
+                    error_num=1,
+                    error_msg="User stopped",
+                )
+
     def _handle_pod_message(self, msg: PodMessage) -> None:
         """Handles a PodMessage. This is a message that signals the completion of a
         prior step Job within an existing running workflow.