diff --git a/simpleflow/history.py b/simpleflow/history.py
index ccc3dd95d..7e79dfe04 100644
--- a/simpleflow/history.py
+++ b/simpleflow/history.py
@@ -4,10 +4,9 @@
 class History(object):
     def __init__(self, history):
         self._history = history
-        self._activities = collections.defaultdict(
-            lambda: {'type': 'activity'})
-        self._child_workflows = collections.defaultdict(
-            lambda: {'type': 'child_workflow'})
+        self._activities = collections.OrderedDict()
+        self._child_workflows = collections.OrderedDict()
+        self._tasks = collections.OrderedDict()
 
     @property
     def events(self):
@@ -31,7 +30,9 @@ def get_activity(event):
                 'scheduled_id': event.id,
             }
             if event.activity_id not in self._activities:
-                self._activities[event.activity_id] = activity
+                id_ = event.activity_id
+                self._activities[id_] = activity
+                self._tasks[id_] = self._activities[id_]
             else:
                 # When the executor retries a task, it schedules it again.
                 # We have to take care of not overriding some values set by the
@@ -40,10 +41,12 @@ def get_activity(event):
                 # corresponds to the last execution.
                 self._activities[event.activity_id].update(activity)
         elif event.state == 'schedule_failed':
-            activity = self._activities[event.activity_id]
-            activity['state'] = event.state
-            activity['cause'] = event.cause
-            activity['activity_type'] = event.activity_type.copy()
+            self._activities[event.activity_id] = {
+                'type': 'activity',
+                'state': event.state,
+                'cause': event.cause,
+                'activity_type': event.activity_type.copy(),
+            }
         elif event.state == 'started':
             activity = get_activity(event)
             activity['state'] = event.state
@@ -106,7 +109,9 @@ def get_workflow(event):
                 'state': event.state,
                 'initiated_event_id': event.id,
             }
-            self._child_workflows[event.workflow_id] = workflow
+            id_ = event.workflow_id
+            self._child_workflows[id_] = workflow
+            self._tasks[id_] = self._child_workflows[id_]
         elif event.state == 'started':
             workflow = get_workflow(event)
             workflow['state'] = event.state
diff --git a/simpleflow/swf/executor.py b/simpleflow/swf/executor.py
index cf7d20e06..2c2d020cc 100644
--- a/simpleflow/swf/executor.py
+++ b/simpleflow/swf/executor.py
@@ -273,6 +273,26 @@ def starmap(self, callable, iterable):
         iterable = executor.get_actual_value(iterable)
         return super(Executor, self).starmap(callable, iterable)
 
+    def merge_previous_execution(self, execution):
+        previous_history = History(execution.history())
+        # Override input with the previous execution value.
+        self._history.events[0].input = json.dumps(
+            previous_history.events[0].input.copy(),
+        )
+
+        # Override already completed tasks to not execute them again.
+        previous_history.parse()
+        self._history._activities.update({
+            id_: activity for id_, activity in
+            previous_history._activities.iteritems() if
+            activity['state'] == 'completed'
+        })
+        self._history._child_workflows.update({
+            id_: child_workflow for id_, child_workflow in
+            previous_history._child_workflows.iteritems() if
+            child_workflow['state'] == 'completed'
+        })
+
     def replay(self, history):
         """Executes the workflow from the start until it blocks.
 
@@ -291,6 +311,17 @@ def replay(self, history):
         args = input.get('args', ())
         kwargs = input.get('kwargs', {})
 
+        previous_workflow_execution = input.get('_previous_workflow_execution')
+        if previous_workflow_execution:
+            # Resume previous execution by injecting input and completed task
+            # in the current history.
+            ex = swf.models.WorkflowExecution(
+                domain=self.domain,
+                workflow_id=previous_workflow_execution['workflow_id'],
+                run_id=previous_workflow_execution['run_id'],
+            )
+            self.merge_previous_execution(ex)
+
         try:
             result = self.run_workflow(*args, **kwargs)
         except exceptions.ExecutionBlocked:
diff --git a/tests/test_dataflow.py b/tests/test_dataflow.py
index 7ebc23f0a..059607af4 100644
--- a/tests/test_dataflow.py
+++ b/tests/test_dataflow.py
@@ -1,5 +1,6 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
+from __future__ import absolute_import
 
 import functools
 import mock
@@ -1006,3 +1007,46 @@ def test_activity_not_found_schedule_failed_already_exists():
         decisions, _ = executor.replay(history)
 
     check_task_scheduled_decision(decisions[0], increment)
+
+
+def test_resume_stopped_workflow_execution():
+    workflow = TestDefinition
+    executor = Executor(DOMAIN, workflow)
+
+    previous_history = builder.History(workflow)
+    decision_id = previous_history.last_id
+    (previous_history
+        .add_activity_task(
+            increment,
+            decision_id=decision_id,
+            last_state='completed',
+            activity_id='activity-tests.test_dataflow.increment-1',
+            input={'args': 1},
+            result=2)
+        .add_decision_task_scheduled()
+        .add_decision_task_started())
+
+    history = builder.History(
+        workflow,
+        input={
+            '_previous_workflow_execution': {
+                'workflow_id': 'WORKFLOW_ID',
+                'run_id': 'RUN_ID',
+            }
+        })
+    decision_id = previous_history.last_id
+    (history
+        .add_decision_task_scheduled()
+        .add_decision_task_started())
+
+    class FakeWorkflowExecution(object):
+        def __init__(self, *args, **kwargs):
+            pass
+
+        def history(self):
+            return previous_history
+
+    with mock.patch('swf.models.WorkflowExecution') as Mock:
+        Mock.return_value = FakeWorkflowExecution()
+        decisions, _ = executor.replay(history)
+        check_task_scheduled_decision(decisions[0], double)