diff --git a/changelog.d/2970.feature b/changelog.d/2970.feature
new file mode 100644
index 0000000000..5eb928563f
--- /dev/null
+++ b/changelog.d/2970.feature
@@ -0,0 +1 @@
+add support for the lazy_loaded_members filter as per MSC1227
diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index 25346baa87..7e767b9bf5 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -113,7 +113,10 @@ ROOM_EVENT_FILTER_SCHEMA = {
         },
         "contains_url": {
             "type": "boolean"
-        }
+        },
+        "lazy_load_members": {
+            "type": "boolean"
+        },
     }
 }
 
@@ -261,6 +264,9 @@ class FilterCollection(object):
     def ephemeral_limit(self):
         return self._room_ephemeral_filter.limit()
 
+    def lazy_load_members(self):
+        return self._room_state_filter.lazy_load_members()
+
     def filter_presence(self, events):
         return self._presence_filter.filter(events)
 
@@ -417,6 +423,9 @@ class Filter(object):
     def limit(self):
         return self.filter_json.get("limit", 10)
 
+    def lazy_load_members(self):
+        return self.filter_json.get("lazy_load_members", False)
+
 
 def _matches_wildcard(actual_value, filter_value):
     if filter_value.endswith("*"):
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index c24e35362a..4ced3144c8 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
-# Copyright 2015 - 2016 OpenMarket Ltd
+# Copyright 2015, 2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -416,29 +417,44 @@ class SyncHandler(object):
         ))
 
     @defer.inlineCallbacks
-    def get_state_after_event(self, event):
+    def get_state_after_event(self, event, types=None, filtered_types=None):
         """
         Get the room state after the given event
 
         Args:
             event(synapse.events.EventBase): event of interest
+            types(list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+                May be None, which matches any key.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             A Deferred map from ((type, state_key)->Event)
         """
-        state_ids = yield self.store.get_state_ids_for_event(event.event_id)
+        state_ids = yield self.store.get_state_ids_for_event(
+            event.event_id, types, filtered_types=filtered_types,
+        )
         if event.is_state():
             state_ids = state_ids.copy()
             state_ids[(event.type, event.state_key)] = event.event_id
         defer.returnValue(state_ids)
 
     @defer.inlineCallbacks
-    def get_state_at(self, room_id, stream_position):
+    def get_state_at(self, room_id, stream_position, types=None, filtered_types=None):
         """ Get the room state at a particular stream position
 
         Args:
             room_id(str): room for which to get state
             stream_position(StreamToken): point at which to get state
+            types(list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             A Deferred map from ((type, state_key)->Event)
@@ -453,7 +469,9 @@ class SyncHandler(object):
 
         if last_events:
             last_event = last_events[-1]
-            state = yield self.get_state_after_event(last_event)
+            state = yield self.get_state_after_event(
+                last_event, types, filtered_types=filtered_types,
+            )
 
         else:
             # no events in this room - so presumably no state
@@ -485,18 +503,42 @@ class SyncHandler(object):
         # TODO(mjark) Check for new redactions in the state events.
 
         with Measure(self.clock, "compute_state_delta"):
+
+            types = None
+            lazy_load_members = sync_config.filter_collection.lazy_load_members()
+            filtered_types = None
+
+            if lazy_load_members:
+                # We only request state for the members needed to display the
+                # timeline:
+
+                types = [
+                    (EventTypes.Member, state_key)
+                    for state_key in set(
+                        event.sender  # FIXME: we also care about invite targets etc.
+                        for event in batch.events
+                    )
+                ]
+
+                # only apply the filtering to room members
+                filtered_types = [EventTypes.Member]
+
             if full_state:
                 if batch:
                     current_state_ids = yield self.store.get_state_ids_for_event(
-                        batch.events[-1].event_id
+                        batch.events[-1].event_id, types=types,
+                        filtered_types=filtered_types,
                     )
 
                     state_ids = yield self.store.get_state_ids_for_event(
-                        batch.events[0].event_id
+                        batch.events[0].event_id, types=types,
+                        filtered_types=filtered_types,
                     )
+
                 else:
                     current_state_ids = yield self.get_state_at(
-                        room_id, stream_position=now_token
+                        room_id, stream_position=now_token, types=types,
+                        filtered_types=filtered_types,
                     )
 
                     state_ids = current_state_ids
@@ -511,18 +553,22 @@ class SyncHandler(object):
                     timeline_start=state_ids,
                     previous={},
                     current=current_state_ids,
+                    lazy_load_members=lazy_load_members,
                 )
             elif batch.limited:
                 state_at_previous_sync = yield self.get_state_at(
-                    room_id, stream_position=since_token
+                    room_id, stream_position=since_token, types=types,
+                    filtered_types=filtered_types,
                 )
 
                 current_state_ids = yield self.store.get_state_ids_for_event(
-                    batch.events[-1].event_id
+                    batch.events[-1].event_id, types=types,
+                    filtered_types=filtered_types,
                 )
 
                 state_at_timeline_start = yield self.store.get_state_ids_for_event(
-                    batch.events[0].event_id
+                    batch.events[0].event_id, types=types,
+                    filtered_types=filtered_types,
                 )
 
                 timeline_state = {
@@ -530,14 +576,35 @@ class SyncHandler(object):
                     for event in batch.events if event.is_state()
                 }
 
+                # TODO: optionally filter out redundant membership events at this
+                # point, to stop repeatedly sending members in every /sync as if
+                # the client isn't tracking them.
+                # When implemented, this should filter using event_ids (not mxids).
+                # In practice, limited syncs are
+                # relatively rare so it's not a total disaster to send redundant
+                # members down at this point. Redundant members are ones which
+                # repeatedly get sent down /sync because we don't know if the client
+                # is caching them or not.
+
                 state_ids = _calculate_state(
                     timeline_contains=timeline_state,
                     timeline_start=state_at_timeline_start,
                     previous=state_at_previous_sync,
                     current=current_state_ids,
+                    lazy_load_members=lazy_load_members,
                 )
             else:
                 state_ids = {}
+                if lazy_load_members:
+                    # TODO: filter out redundant members based on their mxids (not their
+                    # event_ids) at this point. We know we can do it based on mxid as this
+                    # is an non-gappy incremental sync.
+
+                    if types:
+                        state_ids = yield self.store.get_state_ids_for_event(
+                            batch.events[0].event_id, types=types,
+                            filtered_types=filtered_types,
+                        )
 
         state = {}
         if state_ids:
@@ -1448,7 +1515,9 @@ def _action_has_highlight(actions):
     return False
 
 
-def _calculate_state(timeline_contains, timeline_start, previous, current):
+def _calculate_state(
+    timeline_contains, timeline_start, previous, current, lazy_load_members,
+):
     """Works out what state to include in a sync response.
 
     Args:
@@ -1457,6 +1526,9 @@ def _calculate_state(timeline_contains, timeline_start, previous, current):
         previous (dict): state at the end of the previous sync (or empty dict
             if this is an initial sync)
         current (dict): state at the end of the timeline
+        lazy_load_members (bool): whether to return members from timeline_start
+            or not.  assumes that timeline_start has already been filtered to
+            include only the members the client needs to know about.
 
     Returns:
         dict
@@ -1472,9 +1544,25 @@ def _calculate_state(timeline_contains, timeline_start, previous, current):
     }
 
     c_ids = set(e for e in current.values())
-    tc_ids = set(e for e in timeline_contains.values())
-    p_ids = set(e for e in previous.values())
     ts_ids = set(e for e in timeline_start.values())
+    p_ids = set(e for e in previous.values())
+    tc_ids = set(e for e in timeline_contains.values())
+
+    # If we are lazyloading room members, we explicitly add the membership events
+    # for the senders in the timeline into the state block returned by /sync,
+    # as we may not have sent them to the client before.  We find these membership
+    # events by filtering them out of timeline_start, which has already been filtered
+    # to only include membership events for the senders in the timeline.
+    # In practice, we can do this by removing them from the p_ids list,
+    # which is the list of relevant state we know we have already sent to the client.
+    # see https://github.com/matrix-org/synapse/pull/2970
+    #            /files/efcdacad7d1b7f52f879179701c7e0d9b763511f#r204732809
+
+    if lazy_load_members:
+        p_ids.difference_update(
+            e for t, e in timeline_start.iteritems()
+            if t[0] == EventTypes.Member
+        )
 
     state_ids = ((c_ids | ts_ids) - p_ids) - tc_ids
 
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 89a05c4618..b27b3ae144 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -186,7 +186,17 @@ class StateGroupWorkerStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def _get_state_groups_from_groups(self, groups, types):
-        """Returns dictionary state_group -> (dict of (type, state_key) -> event id)
+        """Returns the state groups for a given set of groups, filtering on
+        types of state events.
+
+        Args:
+            groups(list[int]): list of state group IDs to query
+            types (Iterable[str, str|None]|None): list of 2-tuples of the form
+                (`type`, `state_key`), where a `state_key` of `None` matches all
+                state_keys for the `type`. If None, all types are returned.
+
+        Returns:
+            dictionary state_group -> (dict of (type, state_key) -> event id)
         """
         results = {}
 
@@ -200,8 +210,11 @@ class StateGroupWorkerStore(SQLBaseStore):
 
         defer.returnValue(results)
 
-    def _get_state_groups_from_groups_txn(self, txn, groups, types=None):
+    def _get_state_groups_from_groups_txn(
+        self, txn, groups, types=None,
+    ):
         results = {group: {} for group in groups}
+
         if types is not None:
             types = list(set(types))  # deduplicate types list
 
@@ -239,7 +252,7 @@ class StateGroupWorkerStore(SQLBaseStore):
             # Turns out that postgres doesn't like doing a list of OR's and
             # is about 1000x slower, so we just issue a query for each specific
             # type seperately.
-            if types:
+            if types is not None:
                 clause_to_args = [
                     (
                         "AND type = ? AND state_key = ?",
@@ -278,6 +291,7 @@ class StateGroupWorkerStore(SQLBaseStore):
                     else:
                         where_clauses.append("(type = ? AND state_key = ?)")
                         where_args.extend([typ[0], typ[1]])
+
                 where_clause = "AND (%s)" % (" OR ".join(where_clauses))
             else:
                 where_clause = ""
@@ -332,16 +346,20 @@ class StateGroupWorkerStore(SQLBaseStore):
         return results
 
     @defer.inlineCallbacks
-    def get_state_for_events(self, event_ids, types):
+    def get_state_for_events(self, event_ids, types, filtered_types=None):
         """Given a list of event_ids and type tuples, return a list of state
         dicts for each event. The state dicts will only have the type/state_keys
         that are in the `types` list.
 
         Args:
-            event_ids (list)
-            types (list): List of (type, state_key) tuples which are used to
-                filter the state fetched. `state_key` may be None, which matches
-                any `state_key`
+            event_ids (list[string])
+            types (list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+                May be None, which matches any key.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             deferred: A list of dicts corresponding to the event_ids given.
@@ -352,7 +370,7 @@ class StateGroupWorkerStore(SQLBaseStore):
         )
 
         groups = set(itervalues(event_to_groups))
-        group_to_state = yield self._get_state_for_groups(groups, types)
+        group_to_state = yield self._get_state_for_groups(groups, types, filtered_types)
 
         state_event_map = yield self.get_events(
             [ev_id for sd in itervalues(group_to_state) for ev_id in itervalues(sd)],
@@ -371,15 +389,19 @@ class StateGroupWorkerStore(SQLBaseStore):
         defer.returnValue({event: event_to_state[event] for event in event_ids})
 
     @defer.inlineCallbacks
-    def get_state_ids_for_events(self, event_ids, types=None):
+    def get_state_ids_for_events(self, event_ids, types=None, filtered_types=None):
         """
         Get the state dicts corresponding to a list of events
 
         Args:
             event_ids(list(str)): events whose state should be returned
-            types(list[(str, str)]|None): List of (type, state_key) tuples
-                which are used to filter the state fetched. May be None, which
-                matches any key
+            types(list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+                May be None, which matches any key.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             A deferred dict from event_id -> (type, state_key) -> state_event
@@ -389,7 +411,7 @@ class StateGroupWorkerStore(SQLBaseStore):
         )
 
         groups = set(itervalues(event_to_groups))
-        group_to_state = yield self._get_state_for_groups(groups, types)
+        group_to_state = yield self._get_state_for_groups(groups, types, filtered_types)
 
         event_to_state = {
             event_id: group_to_state[group]
@@ -399,37 +421,45 @@ class StateGroupWorkerStore(SQLBaseStore):
         defer.returnValue({event: event_to_state[event] for event in event_ids})
 
     @defer.inlineCallbacks
-    def get_state_for_event(self, event_id, types=None):
+    def get_state_for_event(self, event_id, types=None, filtered_types=None):
         """
         Get the state dict corresponding to a particular event
 
         Args:
             event_id(str): event whose state should be returned
-            types(list[(str, str)]|None): List of (type, state_key) tuples
-                which are used to filter the state fetched. May be None, which
-                matches any key
+            types(list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+                May be None, which matches any key.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             A deferred dict from (type, state_key) -> state_event
         """
-        state_map = yield self.get_state_for_events([event_id], types)
+        state_map = yield self.get_state_for_events([event_id], types, filtered_types)
         defer.returnValue(state_map[event_id])
 
     @defer.inlineCallbacks
-    def get_state_ids_for_event(self, event_id, types=None):
+    def get_state_ids_for_event(self, event_id, types=None, filtered_types=None):
         """
         Get the state dict corresponding to a particular event
 
         Args:
             event_id(str): event whose state should be returned
-            types(list[(str, str)]|None): List of (type, state_key) tuples
-                which are used to filter the state fetched. May be None, which
-                matches any key
+            types(list[(str, str|None)]|None): List of (type, state_key) tuples
+                which are used to filter the state fetched. If `state_key` is None,
+                all events are returned of the given type.
+                May be None, which matches any key.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             A deferred dict from (type, state_key) -> state_event
         """
-        state_map = yield self.get_state_ids_for_events([event_id], types)
+        state_map = yield self.get_state_ids_for_events([event_id], types, filtered_types)
         defer.returnValue(state_map[event_id])
 
     @cached(max_entries=50000)
@@ -460,56 +490,73 @@ class StateGroupWorkerStore(SQLBaseStore):
 
         defer.returnValue({row["event_id"]: row["state_group"] for row in rows})
 
-    def _get_some_state_from_cache(self, group, types):
+    def _get_some_state_from_cache(self, group, types, filtered_types=None):
         """Checks if group is in cache. See `_get_state_for_groups`
 
-        Returns 3-tuple (`state_dict`, `missing_types`, `got_all`).
-        `missing_types` is the list of types that aren't in the cache for that
-        group. `got_all` is a bool indicating if we successfully retrieved all
+        Args:
+            group(int): The state group to lookup
+            types(list[str, str|None]): List of 2-tuples of the form
+                (`type`, `state_key`), where a `state_key` of `None` matches all
+                state_keys for the `type`.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
+
+        Returns 2-tuple (`state_dict`, `got_all`).
+        `got_all` is a bool indicating if we successfully retrieved all
         requests state from the cache, if False we need to query the DB for the
         missing state.
-
-        Args:
-            group: The state group to lookup
-            types (list): List of 2-tuples of the form (`type`, `state_key`),
-                where a `state_key` of `None` matches all state_keys for the
-                `type`.
         """
         is_all, known_absent, state_dict_ids = self._state_group_cache.get(group)
 
         type_to_key = {}
-        missing_types = set()
+
+        # tracks whether any of ourrequested types are missing from the cache
+        missing_types = False
 
         for typ, state_key in types:
             key = (typ, state_key)
-            if state_key is None:
+
+            if (
+                state_key is None or
+                (filtered_types is not None and typ not in filtered_types)
+            ):
                 type_to_key[typ] = None
-                missing_types.add(key)
+                # we mark the type as missing from the cache because
+                # when the cache was populated it might have been done with a
+                # restricted set of state_keys, so the wildcard will not work
+                # and the cache may be incomplete.
+                missing_types = True
             else:
                 if type_to_key.get(typ, object()) is not None:
                     type_to_key.setdefault(typ, set()).add(state_key)
 
                 if key not in state_dict_ids and key not in known_absent:
-                    missing_types.add(key)
+                    missing_types = True
 
         sentinel = object()
 
         def include(typ, state_key):
             valid_state_keys = type_to_key.get(typ, sentinel)
             if valid_state_keys is sentinel:
-                return False
+                return filtered_types is not None and typ not in filtered_types
             if valid_state_keys is None:
                 return True
             if state_key in valid_state_keys:
                 return True
             return False
 
-        got_all = is_all or not missing_types
+        got_all = is_all
+        if not got_all:
+            # the cache is incomplete. We may still have got all the results we need, if
+            # we don't have any wildcards in the match list.
+            if not missing_types and filtered_types is None:
+                got_all = True
 
         return {
             k: v for k, v in iteritems(state_dict_ids)
             if include(k[0], k[1])
-        }, missing_types, got_all
+        }, got_all
 
     def _get_all_state_from_cache(self, group):
         """Checks if group is in cache. See `_get_state_for_groups`
@@ -526,7 +573,7 @@ class StateGroupWorkerStore(SQLBaseStore):
         return state_dict_ids, is_all
 
     @defer.inlineCallbacks
-    def _get_state_for_groups(self, groups, types=None):
+    def _get_state_for_groups(self, groups, types=None, filtered_types=None):
         """Gets the state at each of a list of state groups, optionally
         filtering by type/state_key
 
@@ -540,6 +587,9 @@ class StateGroupWorkerStore(SQLBaseStore):
                 Otherwise, each entry should be a `(type, state_key)` tuple to
                 include in the response. A `state_key` of None is a wildcard
                 meaning that we require all state with that type.
+            filtered_types(list[str]|None): Only apply filtering via `types` to this
+                list of event types.  Other types of events are returned unfiltered.
+                If None, `types` filtering is applied to all events.
 
         Returns:
             Deferred[dict[int, dict[(type, state_key), EventBase]]]
@@ -551,8 +601,8 @@ class StateGroupWorkerStore(SQLBaseStore):
         missing_groups = []
         if types is not None:
             for group in set(groups):
-                state_dict_ids, _, got_all = self._get_some_state_from_cache(
-                    group, types,
+                state_dict_ids, got_all = self._get_some_state_from_cache(
+                    group, types, filtered_types
                 )
                 results[group] = state_dict_ids
 
@@ -579,13 +629,13 @@ class StateGroupWorkerStore(SQLBaseStore):
             # cache. Hence, if we are doing a wildcard lookup, populate the
             # cache fully so that we can do an efficient lookup next time.
 
-            if types and any(k is None for (t, k) in types):
+            if filtered_types or (types and any(k is None for (t, k) in types)):
                 types_to_fetch = None
             else:
                 types_to_fetch = types
 
             group_to_state_dict = yield self._get_state_groups_from_groups(
-                missing_groups, types_to_fetch,
+                missing_groups, types_to_fetch
             )
 
             for group, group_state_dict in iteritems(group_to_state_dict):
@@ -595,7 +645,10 @@ class StateGroupWorkerStore(SQLBaseStore):
                 if types:
                     for k, v in iteritems(group_state_dict):
                         (typ, _) = k
-                        if k in types or (typ, None) in types:
+                        if (
+                            (k in types or (typ, None) in types) or
+                            (filtered_types and typ not in filtered_types)
+                        ):
                             state_dict[k] = v
                 else:
                     state_dict.update(group_state_dict)
diff --git a/tests/storage/test_state.py b/tests/storage/test_state.py
new file mode 100644
index 0000000000..7a76d67b8c
--- /dev/null
+++ b/tests/storage/test_state.py
@@ -0,0 +1,319 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+from twisted.internet import defer
+
+from synapse.api.constants import EventTypes, Membership
+from synapse.types import RoomID, UserID
+
+import tests.unittest
+import tests.utils
+
+logger = logging.getLogger(__name__)
+
+
+class StateStoreTestCase(tests.unittest.TestCase):
+    def __init__(self, *args, **kwargs):
+        super(StateStoreTestCase, self).__init__(*args, **kwargs)
+        self.store = None  # type: synapse.storage.DataStore
+
+    @defer.inlineCallbacks
+    def setUp(self):
+        hs = yield tests.utils.setup_test_homeserver()
+
+        self.store = hs.get_datastore()
+        self.event_builder_factory = hs.get_event_builder_factory()
+        self.event_creation_handler = hs.get_event_creation_handler()
+
+        self.u_alice = UserID.from_string("@alice:test")
+        self.u_bob = UserID.from_string("@bob:test")
+
+        self.room = RoomID.from_string("!abc123:test")
+
+        yield self.store.store_room(
+            self.room.to_string(),
+            room_creator_user_id="@creator:text",
+            is_public=True
+        )
+
+    @defer.inlineCallbacks
+    def inject_state_event(self, room, sender, typ, state_key, content):
+        builder = self.event_builder_factory.new({
+            "type": typ,
+            "sender": sender.to_string(),
+            "state_key": state_key,
+            "room_id": room.to_string(),
+            "content": content,
+        })
+
+        event, context = yield self.event_creation_handler.create_new_client_event(
+            builder
+        )
+
+        yield self.store.persist_event(event, context)
+
+        defer.returnValue(event)
+
+    def assertStateMapEqual(self, s1, s2):
+        for t in s1:
+            # just compare event IDs for simplicity
+            self.assertEqual(s1[t].event_id, s2[t].event_id)
+        self.assertEqual(len(s1), len(s2))
+
+    @defer.inlineCallbacks
+    def test_get_state_for_event(self):
+
+        # this defaults to a linear DAG as each new injection defaults to whatever
+        # forward extremities are currently in the DB for this room.
+        e1 = yield self.inject_state_event(
+            self.room, self.u_alice, EventTypes.Create, '', {},
+        )
+        e2 = yield self.inject_state_event(
+            self.room, self.u_alice, EventTypes.Name, '', {
+                "name": "test room"
+            },
+        )
+        e3 = yield self.inject_state_event(
+            self.room, self.u_alice, EventTypes.Member, self.u_alice.to_string(), {
+                "membership": Membership.JOIN
+            },
+        )
+        e4 = yield self.inject_state_event(
+            self.room, self.u_bob, EventTypes.Member, self.u_bob.to_string(), {
+                "membership": Membership.JOIN
+            },
+        )
+        e5 = yield self.inject_state_event(
+            self.room, self.u_bob, EventTypes.Member, self.u_bob.to_string(), {
+                "membership": Membership.LEAVE
+            },
+        )
+
+        # check we get the full state as of the final event
+        state = yield self.store.get_state_for_event(
+            e5.event_id, None, filtered_types=None
+        )
+
+        self.assertIsNotNone(e4)
+
+        self.assertStateMapEqual({
+            (e1.type, e1.state_key): e1,
+            (e2.type, e2.state_key): e2,
+            (e3.type, e3.state_key): e3,
+            # e4 is overwritten by e5
+            (e5.type, e5.state_key): e5,
+        }, state)
+
+        # check we can filter to the m.room.name event (with a '' state key)
+        state = yield self.store.get_state_for_event(
+            e5.event_id, [(EventTypes.Name, '')], filtered_types=None
+        )
+
+        self.assertStateMapEqual({
+            (e2.type, e2.state_key): e2,
+        }, state)
+
+        # check we can filter to the m.room.name event (with a wildcard None state key)
+        state = yield self.store.get_state_for_event(
+            e5.event_id, [(EventTypes.Name, None)], filtered_types=None
+        )
+
+        self.assertStateMapEqual({
+            (e2.type, e2.state_key): e2,
+        }, state)
+
+        # check we can grab the m.room.member events (with a wildcard None state key)
+        state = yield self.store.get_state_for_event(
+            e5.event_id, [(EventTypes.Member, None)], filtered_types=None
+        )
+
+        self.assertStateMapEqual({
+            (e3.type, e3.state_key): e3,
+            (e5.type, e5.state_key): e5,
+        }, state)
+
+        # check we can use filter_types to grab a specific room member
+        # without filtering out the other event types
+        state = yield self.store.get_state_for_event(
+            e5.event_id, [(EventTypes.Member, self.u_alice.to_string())],
+            filtered_types=[EventTypes.Member],
+        )
+
+        self.assertStateMapEqual({
+            (e1.type, e1.state_key): e1,
+            (e2.type, e2.state_key): e2,
+            (e3.type, e3.state_key): e3,
+        }, state)
+
+        # check that types=[], filtered_types=[EventTypes.Member]
+        # doesn't return all members
+        state = yield self.store.get_state_for_event(
+            e5.event_id, [], filtered_types=[EventTypes.Member],
+        )
+
+        self.assertStateMapEqual({
+            (e1.type, e1.state_key): e1,
+            (e2.type, e2.state_key): e2,
+        }, state)
+
+        #######################################################
+        # _get_some_state_from_cache tests against a full cache
+        #######################################################
+
+        room_id = self.room.to_string()
+        group_ids = yield self.store.get_state_groups_ids(room_id, [e5.event_id])
+        group = group_ids.keys()[0]
+
+        # test _get_some_state_from_cache correctly filters out members with types=[]
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, True)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+            (e2.type, e2.state_key): e2.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members with wildcard types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, None)], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, True)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+            (e2.type, e2.state_key): e2.event_id,
+            (e3.type, e3.state_key): e3.event_id,
+            # e4 is overwritten by e5
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members with specific types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, e5.state_key)], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, True)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+            (e2.type, e2.state_key): e2.event_id,
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members with specific types
+        # and no filtered_types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, e5.state_key)], filtered_types=None
+        )
+
+        self.assertEqual(is_all, True)
+        self.assertDictEqual({
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)
+
+        #######################################################
+        # deliberately remove e2 (room name) from the _state_group_cache
+
+        (is_all, known_absent, state_dict_ids) = self.store._state_group_cache.get(group)
+
+        self.assertEqual(is_all, True)
+        self.assertEqual(known_absent, set())
+        self.assertDictEqual(state_dict_ids, {
+            (e1.type, e1.state_key): e1.event_id,
+            (e2.type, e2.state_key): e2.event_id,
+            (e3.type, e3.state_key): e3.event_id,
+            # e4 is overwritten by e5
+            (e5.type, e5.state_key): e5.event_id,
+        })
+
+        state_dict_ids.pop((e2.type, e2.state_key))
+        self.store._state_group_cache.invalidate(group)
+        self.store._state_group_cache.update(
+            sequence=self.store._state_group_cache.sequence,
+            key=group,
+            value=state_dict_ids,
+            # list fetched keys so it knows it's partial
+            fetched_keys=(
+                (e1.type, e1.state_key),
+                (e3.type, e3.state_key),
+                (e5.type, e5.state_key),
+            )
+        )
+
+        (is_all, known_absent, state_dict_ids) = self.store._state_group_cache.get(group)
+
+        self.assertEqual(is_all, False)
+        self.assertEqual(known_absent, set([
+            (e1.type, e1.state_key),
+            (e3.type, e3.state_key),
+            (e5.type, e5.state_key),
+        ]))
+        self.assertDictEqual(state_dict_ids, {
+            (e1.type, e1.state_key): e1.event_id,
+            (e3.type, e3.state_key): e3.event_id,
+            (e5.type, e5.state_key): e5.event_id,
+        })
+
+        ############################################
+        # test that things work with a partial cache
+
+        # test _get_some_state_from_cache correctly filters out members with types=[]
+        room_id = self.room.to_string()
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, False)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members wildcard types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, None)], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, False)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+            (e3.type, e3.state_key): e3.event_id,
+            # e4 is overwritten by e5
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members with specific types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, e5.state_key)], filtered_types=[EventTypes.Member]
+        )
+
+        self.assertEqual(is_all, False)
+        self.assertDictEqual({
+            (e1.type, e1.state_key): e1.event_id,
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)
+
+        # test _get_some_state_from_cache correctly filters in members with specific types
+        # and no filtered_types
+        (state_dict, is_all) = yield self.store._get_some_state_from_cache(
+            group, [(EventTypes.Member, e5.state_key)], filtered_types=None
+        )
+
+        self.assertEqual(is_all, True)
+        self.assertDictEqual({
+            (e5.type, e5.state_key): e5.event_id,
+        }, state_dict)