diff --git a/wagtail/api/v2/tests/test_pages.py b/wagtail/api/v2/tests/test_pages.py
index 1851649592..a84bc265b3 100644
--- a/wagtail/api/v2/tests/test_pages.py
+++ b/wagtail/api/v2/tests/test_pages.py
@@ -1026,7 +1026,10 @@ class TestPageDetailWithStreamField(TestCase):
         self.assertIn('id', content)
         self.assertEqual(content['id'], stream_page.id)
         self.assertIn('body', content)
-        self.assertEqual(content['body'], [{'type': 'text', 'value': 'foo'}])
+        self.assertEqual(len(content['body']), 1)
+        self.assertEqual(content['body'][0]['type'], 'text')
+        self.assertEqual(content['body'][0]['value'], 'foo')
+        self.assertTrue(content['body'][0]['id'])
 
     def test_image_block(self):
         stream_page = self.make_stream_page('[{"type": "image", "value": 1}]')
@@ -1036,7 +1039,8 @@ class TestPageDetailWithStreamField(TestCase):
         content = json.loads(response.content.decode('utf-8'))
 
         # ForeignKeys in a StreamField shouldn't be translated into dictionary representation
-        self.assertEqual(content['body'], [{'type': 'image', 'value': 1}])
+        self.assertEqual(content['body'][0]['type'], 'image')
+        self.assertEqual(content['body'][0]['value'], 1)
 
     def test_image_block_with_custom_get_api_representation(self):
         stream_page = self.make_stream_page('[{"type": "image", "value": 1}]')
@@ -1048,7 +1052,8 @@ class TestPageDetailWithStreamField(TestCase):
         content = json.loads(response.content.decode('utf-8'))
 
         # the custom get_api_representation returns a dict of id and title for the image
-        self.assertEqual(content['body'], [{'type': 'image', 'value': {'id': 1, 'title': 'A missing image'}}])
+        self.assertEqual(content['body'][0]['type'], 'image')
+        self.assertEqual(content['body'][0]['value'], {'id': 1, 'title': 'A missing image'})
 
 
 @override_settings(
diff --git a/wagtail/contrib/wagtailapi/tests/test_pages.py b/wagtail/contrib/wagtailapi/tests/test_pages.py
index 3c838a20ca..980746a22f 100644
--- a/wagtail/contrib/wagtailapi/tests/test_pages.py
+++ b/wagtail/contrib/wagtailapi/tests/test_pages.py
@@ -677,7 +677,10 @@ class TestPageDetailWithStreamField(TestCase):
         self.assertIn('id', content)
         self.assertEqual(content['id'], stream_page.id)
         self.assertIn('body', content)
-        self.assertEqual(content['body'], [{'type': 'text', 'value': 'foo'}])
+        self.assertEqual(len(content['body']), 1)
+        self.assertEqual(content['body'][0]['type'], 'text')
+        self.assertEqual(content['body'][0]['value'], 'foo')
+        self.assertTrue(content['body'][0]['id'])
 
     def test_image_block(self):
         stream_page = self.make_stream_page('[{"type": "image", "value": 1}]')
@@ -687,7 +690,8 @@ class TestPageDetailWithStreamField(TestCase):
         content = json.loads(response.content.decode('utf-8'))
 
         # ForeignKeys in a StreamField shouldn't be translated into dictionary representation
-        self.assertEqual(content['body'], [{'type': 'image', 'value': 1}])
+        self.assertEqual(content['body'][0]['type'], 'image')
+        self.assertEqual(content['body'][0]['value'], 1)
 
 
 @override_settings(
diff --git a/wagtail/wagtailadmin/templates/wagtailadmin/block_forms/stream_member.html b/wagtail/wagtailadmin/templates/wagtailadmin/block_forms/stream_member.html
index 027ebde6b0..18cf4f74b3 100644
--- a/wagtail/wagtailadmin/templates/wagtailadmin/block_forms/stream_member.html
+++ b/wagtail/wagtailadmin/templates/wagtailadmin/block_forms/stream_member.html
@@ -3,6 +3,7 @@
 
 {% block hidden_fields %}
     <input type="hidden" id="{{ prefix }}-type" name="{{ prefix }}-type" value="{{ child.block.name }}">
+    <input type="hidden" id="{{ prefix }}-id" name="{{ prefix }}-id" value="{{ block_id|default:"" }}">
 {% endblock %}
 
 {% block header_controls %}
diff --git a/wagtail/wagtailadmin/tests/api/test_pages.py b/wagtail/wagtailadmin/tests/api/test_pages.py
index 8c5d03b697..9c35145d1b 100644
--- a/wagtail/wagtailadmin/tests/api/test_pages.py
+++ b/wagtail/wagtailadmin/tests/api/test_pages.py
@@ -568,7 +568,10 @@ class TestAdminPageDetailWithStreamField(AdminAPITestCase):
         self.assertIn('id', content)
         self.assertEqual(content['id'], stream_page.id)
         self.assertIn('body', content)
-        self.assertEqual(content['body'], [{'type': 'text', 'value': 'foo'}])
+        self.assertEqual(len(content['body']), 1)
+        self.assertEqual(content['body'][0]['type'], 'text')
+        self.assertEqual(content['body'][0]['value'], 'foo')
+        self.assertTrue(content['body'][0]['id'])
 
     def test_image_block(self):
         stream_page = self.make_stream_page('[{"type": "image", "value": 1}]')
@@ -578,4 +581,5 @@ class TestAdminPageDetailWithStreamField(AdminAPITestCase):
         content = json.loads(response.content.decode('utf-8'))
 
         # ForeignKeys in a StreamField shouldn't be translated into dictionary representation
-        self.assertEqual(content['body'], [{'type': 'image', 'value': 1}])
+        self.assertEqual(content['body'][0]['type'], 'image')
+        self.assertEqual(content['body'][0]['value'], 1)
diff --git a/wagtail/wagtailcore/blocks/stream_block.py b/wagtail/wagtailcore/blocks/stream_block.py
index 32277d6f65..6c9093fdb4 100644
--- a/wagtail/wagtailcore/blocks/stream_block.py
+++ b/wagtail/wagtailcore/blocks/stream_block.py
@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 import collections
+import uuid
 
 from django import forms
 from django.contrib.staticfiles.templatetags.staticfiles import static
@@ -59,7 +60,7 @@ class BaseStreamBlock(Block):
         """
         return StreamValue(self, self.meta.default)
 
-    def render_list_member(self, block_type_name, value, prefix, index, errors=None):
+    def render_list_member(self, block_type_name, value, prefix, index, errors=None, id=None):
         """
         Render the HTML for a single list item. This consists of an <li> wrapper, hidden fields
         to manage ID/deleted state/type, delete/reorder buttons, and the child block's own HTML.
@@ -72,6 +73,7 @@ class BaseStreamBlock(Block):
             'prefix': prefix,
             'child': child,
             'index': index,
+            'block_id': id,
         })
 
     def html_declarations(self):
@@ -133,7 +135,7 @@ class BaseStreamBlock(Block):
 
         list_members_html = [
             self.render_list_member(child.block_type, child.value, "%s-%d" % (prefix, i), i,
-                                    errors=error_dict.get(i))
+                                    errors=error_dict.get(i), id=child.id)
             for (i, child) in enumerate(valid_children)
         ]
 
@@ -162,13 +164,14 @@ class BaseStreamBlock(Block):
                     int(data['%s-%d-order' % (prefix, i)]),
                     block_type_name,
                     child_block.value_from_datadict(data, files, '%s-%d-value' % (prefix, i)),
+                    data.get('%s-%d-id' % (prefix, i)),
                 )
             )
 
         values_with_indexes.sort()
         return StreamValue(self, [
-            (child_block_type_name, value)
-            for (index, child_block_type_name, value) in values_with_indexes
+            (child_block_type_name, value, block_id)
+            for (index, child_block_type_name, value, block_id) in values_with_indexes
         ])
 
     def value_omitted_from_data(self, data, files, prefix):
@@ -177,10 +180,10 @@ class BaseStreamBlock(Block):
     def clean(self, value):
         cleaned_data = []
         errors = {}
-        for i, child in enumerate(value):  # child is a BoundBlock instance
+        for i, child in enumerate(value):  # child is a StreamChild instance
             try:
                 cleaned_data.append(
-                    (child.block.name, child.block.clean(child.value))
+                    (child.block.name, child.block.clean(child.value), child.id)
                 )
             except ValidationError as e:
                 errors[i] = ErrorList([e])
@@ -193,7 +196,8 @@ class BaseStreamBlock(Block):
         return StreamValue(self, cleaned_data)
 
     def to_python(self, value):
-        # the incoming JSONish representation is a list of dicts, each with a 'type' and 'value' field.
+        # the incoming JSONish representation is a list of dicts, each with a 'type' and 'value' field
+        # (and possibly an 'id' too).
         # This is passed to StreamValue to be expanded lazily - but first we reject any unrecognised
         # block types from the list
         return StreamValue(self, [
@@ -207,8 +211,13 @@ class BaseStreamBlock(Block):
             return []
 
         return [
-            {'type': child.block.name, 'value': child.block.get_prep_value(child.value)}
-            for child in value  # child is a BoundBlock instance
+            {
+                'type': child.block.name,
+                'value': child.block.get_prep_value(child.value),
+                # assign a new ID on save if it didn't have one already
+                'id': child.id or str(uuid.uuid4()),
+            }
+            for child in value  # child is a StreamChild instance
         ]
 
     def get_api_representation(self, value, context=None):
@@ -217,8 +226,12 @@ class BaseStreamBlock(Block):
             return []
 
         return [
-            {'type': child.block.name, 'value': child.block.get_api_representation(child.value, context=context)}
-            for child in value  # child is a BoundBlock instance
+            {
+                'type': child.block.name,
+                'value': child.block.get_api_representation(child.value, context=context),
+                'id': child.id
+            }
+            for child in value  # child is a StreamChild instance
         ]
 
     def render_basic(self, value, context=None):
@@ -285,6 +298,10 @@ class StreamValue(collections.Sequence):
         children of StreamField, but not necessarily elsewhere that BoundBlock is used
         """
 
+        def __init__(self, *args, **kwargs):
+            self.id = kwargs.pop('id')
+            super(StreamValue.StreamChild, self).__init__(*args, **kwargs)
+
         @property
         def block_type(self):
             """
@@ -307,7 +324,7 @@ class StreamValue(collections.Sequence):
 
         Passing is_lazy=False means that stream_data consists of immediately usable
         native values. In this mode, stream_data is a list of (type_name, value)
-        tuples.
+        or (type_name, value, id) tuples.
 
         raw_text exists solely as a way of representing StreamField content that is
         not valid JSON; this may legitimately occur if an existing text field is
@@ -332,11 +349,17 @@ class StreamValue(collections.Sequence):
                     return self._bound_blocks[i]
                 else:
                     value = child_block.to_python(raw_value['value'])
+                    block_id = raw_value.get('id')
             else:
-                type_name, value = self.stream_data[i]
+                try:
+                    type_name, value, block_id = self.stream_data[i]
+                except ValueError:
+                    type_name, value = self.stream_data[i]
+                    block_id = None
+
                 child_block = self.stream_block.child_blocks[type_name]
 
-            self._bound_blocks[i] = StreamValue.StreamChild(child_block, value)
+            self._bound_blocks[i] = StreamValue.StreamChild(child_block, value, id=block_id)
 
         return self._bound_blocks[i]
 
@@ -346,14 +369,20 @@ class StreamValue(collections.Sequence):
 
         This prevents n queries for n blocks of a specific type.
         """
+        # create a mapping of all the child blocks matching the given block type,
+        # mapping (index within the stream) => (raw block value)
         raw_values = collections.OrderedDict(
             (i, item['value']) for i, item in enumerate(self.stream_data)
             if item['type'] == type_name
         )
+        # pass the raw block values to bulk_to_python as a list
         converted_values = child_block.bulk_to_python(raw_values.values())
 
+        # reunite the converted values with their stream indexes
         for i, value in zip(raw_values.keys(), converted_values):
-            self._bound_blocks[i] = StreamValue.StreamChild(child_block, value)
+            # also pass the block ID to StreamChild, if one exists for this stream index
+            block_id = self.stream_data[i].get('id')
+            self._bound_blocks[i] = StreamValue.StreamChild(child_block, value, id=block_id)
 
     def __eq__(self, other):
         if not isinstance(other, StreamValue):
diff --git a/wagtail/wagtailcore/tests/test_blocks.py b/wagtail/wagtailcore/tests/test_blocks.py
index faa32795a0..a9c0ad13b1 100644
--- a/wagtail/wagtailcore/tests/test_blocks.py
+++ b/wagtail/wagtailcore/tests/test_blocks.py
@@ -1707,7 +1707,7 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         api_representation = block.get_api_representation(
             block.to_python([
                 {'type': 'language', 'value': 'en'},
-                {'type': 'author', 'value': 'wagtail'},
+                {'type': 'author', 'value': 'wagtail', 'id': '111111'},
             ]),
             context={
                 'en': 'English',
@@ -1717,8 +1717,8 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
 
         self.assertListEqual(
             api_representation, [
-                {'type': 'language', 'value': 'English'},
-                {'type': 'author', 'value': 'Wagtail!'},
+                {'type': 'language', 'value': 'English', 'id': None},
+                {'type': 'author', 'value': 'Wagtail!', 'id': '111111'},
             ]
         )
 
@@ -1846,6 +1846,7 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
             {
                 'type': 'heading',
                 'value': "My title",
+                'id': '123123123',
             },
             {
                 'type': 'paragraph',
@@ -1880,6 +1881,13 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         self.assertIn('<input type="hidden" id="myarticle-1-order" name="myarticle-1-order" value="1">', html)
         self.assertIn('<input type="hidden" id="myarticle-2-order" name="myarticle-2-order" value="2">', html)
 
+    def test_render_form_id_fields(self):
+        html = self.render_form()
+
+        self.assertIn('<input type="hidden" id="myarticle-0-id" name="myarticle-0-id" value="123123123">', html)
+        self.assertIn('<input type="hidden" id="myarticle-1-id" name="myarticle-1-id" value="">', html)
+        self.assertIn('<input type="hidden" id="myarticle-2-id" name="myarticle-2-id" value="">', html)
+
     def test_render_form_type_fields(self):
         html = self.render_form()
 
@@ -1933,24 +1941,12 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
             url = blocks.URLBlock()
         block = ValidatedBlock()
 
-        value = [
-            blocks.BoundBlock(
-                block=block.child_blocks['char'],
-                value='',
-            ),
-            blocks.BoundBlock(
-                block=block.child_blocks['char'],
-                value='foo',
-            ),
-            blocks.BoundBlock(
-                block=block.child_blocks['url'],
-                value='http://example.com/',
-            ),
-            blocks.BoundBlock(
-                block=block.child_blocks['url'],
-                value='not a url',
-            ),
-        ]
+        value = blocks.StreamValue(block, [
+            ('char', ''),
+            ('char', 'foo'),
+            ('url', 'http://example.com/'),
+            ('url', 'not a url'),
+        ])
 
         with self.assertRaises(ValidationError) as catcher:
             block.clean(value)
@@ -2015,6 +2011,8 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         block = ArticleBlock()
         html = block.html_declarations()
 
+        self.assertTagInTemplateScript('<input type="hidden" id="__PREFIX__-id" name="__PREFIX__-id" value="" />', html)
+        self.assertTagInTemplateScript('<input type="hidden" id="__PREFIX__-type" name="__PREFIX__-type" value="heading" />', html)
         self.assertTagInTemplateScript('<input id="__PREFIX__-value" name="__PREFIX__-value" placeholder="Heading" type="text" />', html)
         self.assertTagInTemplateScript(
             '<input id="__PREFIX__-value" name="__PREFIX__-value" placeholder="Paragraph" type="text" />',
@@ -2081,11 +2079,13 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
                 'article-%d-deleted' % i: '',
                 'article-%d-order' % i: str(2 - i),
                 'article-%d-type' % i: 'heading',
-                'article-%d-value' % i: "heading %d" % i
+                'article-%d-value' % i: "heading %d" % i,
+                'article-%d-id' % i: "000%d" % i,
             })
 
         block_value = block.value_from_datadict(post_data, {}, 'article')
         self.assertEqual(block_value[2].value, "heading 0")
+        self.assertEqual(block_value[2].id, "0000")
 
     def test_ordering_in_form_submission_is_numeric(self):
         class ArticleBlock(blocks.StreamBlock):
@@ -2234,6 +2234,62 @@ class TestStreamBlock(WagtailTestUtils, SimpleTestCase):
         self.assertIn('<h3>group1</h3>', html)
         self.assertIn('<h3>group2</h3>', html)
 
+    def test_value_from_datadict(self):
+        class ArticleBlock(blocks.StreamBlock):
+            heading = blocks.CharBlock()
+            paragraph = blocks.CharBlock()
+
+        block = ArticleBlock()
+
+        value = block.value_from_datadict({
+            'foo-count': '3',
+            'foo-0-deleted': '',
+            'foo-0-order': '2',
+            'foo-0-type': 'heading',
+            'foo-0-id': '0000',
+            'foo-0-value': 'this is my heading',
+            'foo-1-deleted': '1',
+            'foo-1-order': '1',
+            'foo-1-type': 'heading',
+            'foo-1-id': '0001',
+            'foo-1-value': 'a deleted heading',
+            'foo-2-deleted': '',
+            'foo-2-order': '0',
+            'foo-2-type': 'paragraph',
+            'foo-2-id': '',
+            'foo-2-value': '<p>this is a paragraph</p>',
+        }, {}, prefix='foo')
+
+        self.assertEqual(len(value), 2)
+        self.assertEqual(value[0].block_type, 'paragraph')
+        self.assertEqual(value[0].id, '')
+        self.assertEqual(value[0].value, '<p>this is a paragraph</p>')
+
+        self.assertEqual(value[1].block_type, 'heading')
+        self.assertEqual(value[1].id, '0000')
+        self.assertEqual(value[1].value, 'this is my heading')
+
+    def test_get_prep_value(self):
+        class ArticleBlock(blocks.StreamBlock):
+            heading = blocks.CharBlock()
+            paragraph = blocks.CharBlock()
+
+        block = ArticleBlock()
+
+        value = blocks.StreamValue(block, [
+            ('heading', 'this is my heading', '0000'),
+            ('paragraph', '<p>this is a paragraph</p>')
+        ])
+        jsonish_value = block.get_prep_value(value)
+
+        self.assertEqual(len(jsonish_value), 2)
+        self.assertEqual(jsonish_value[0], {'type': 'heading', 'value': 'this is my heading', 'id': '0000'})
+        self.assertEqual(jsonish_value[1]['type'], 'paragraph')
+        self.assertEqual(jsonish_value[1]['value'], '<p>this is a paragraph</p>')
+        # get_prep_value should assign a new (random and non-empty) ID to this block, as it didn't
+        # have one already
+        self.assertTrue(jsonish_value[1]['id'])
+
 
 class TestPageChooserBlock(TestCase):
     fixtures = ['test.json']