diff --git a/changelog.d/5232.misc b/changelog.d/5232.misc
index 1cdc71f095..8336bc55dc 100644
--- a/changelog.d/5232.misc
+++ b/changelog.d/5232.misc
@@ -1 +1 @@
-Run black on synapse.crypto.keyring.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5234.misc b/changelog.d/5234.misc
index 43fbd6f67c..8336bc55dc 100644
--- a/changelog.d/5234.misc
+++ b/changelog.d/5234.misc
@@ -1 +1 @@
-Rewrite store_server_verify_key to store several keys at once.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5235.misc b/changelog.d/5235.misc
index 2296ad2a4f..8336bc55dc 100644
--- a/changelog.d/5235.misc
+++ b/changelog.d/5235.misc
@@ -1 +1 @@
-Remove unused VerifyKey.expired and .time_added fields.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5236.misc b/changelog.d/5236.misc
index cb4417a9f4..8336bc55dc 100644
--- a/changelog.d/5236.misc
+++ b/changelog.d/5236.misc
@@ -1 +1 @@
-Simplify Keyring.process_v2_response.
\ No newline at end of file
+Preparatory work for key-validity features.
diff --git a/changelog.d/5237.misc b/changelog.d/5237.misc
index f4fe3b821b..8336bc55dc 100644
--- a/changelog.d/5237.misc
+++ b/changelog.d/5237.misc
@@ -1 +1 @@
-Store key validity time in the storage layer.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5244.misc b/changelog.d/5244.misc
index 9cc1fb869d..8336bc55dc 100644
--- a/changelog.d/5244.misc
+++ b/changelog.d/5244.misc
@@ -1 +1 @@
-Refactor synapse.crypto.keyring to use a KeyFetcher interface.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5250.misc b/changelog.d/5250.misc
index 575a299a82..8336bc55dc 100644
--- a/changelog.d/5250.misc
+++ b/changelog.d/5250.misc
@@ -1 +1 @@
-Simplification to Keyring.wait_for_previous_lookups.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5284.misc b/changelog.d/5284.misc
new file mode 100644
index 0000000000..c4d42ca3d9
--- /dev/null
+++ b/changelog.d/5284.misc
@@ -0,0 +1 @@
+Improve sample config for monthly active user blocking. 
diff --git a/changelog.d/5296.misc b/changelog.d/5296.misc
index a038a6f7f6..8336bc55dc 100644
--- a/changelog.d/5296.misc
+++ b/changelog.d/5296.misc
@@ -1 +1 @@
-Refactor keyring.VerifyKeyRequest to use attr.s.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5299.misc b/changelog.d/5299.misc
index 53297c768b..8336bc55dc 100644
--- a/changelog.d/5299.misc
+++ b/changelog.d/5299.misc
@@ -1 +1 @@
-Rewrite get_server_verify_keys, again.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5317.bugfix b/changelog.d/5317.bugfix
new file mode 100644
index 0000000000..2709375214
--- /dev/null
+++ b/changelog.d/5317.bugfix
@@ -0,0 +1 @@
+Fix handling of failures when processing incoming events where calling `/event_auth` on remote server fails.
diff --git a/changelog.d/5343.misc b/changelog.d/5343.misc
index dbee0f71b9..8336bc55dc 100644
--- a/changelog.d/5343.misc
+++ b/changelog.d/5343.misc
@@ -1 +1 @@
-Rename VerifyKeyRequest.deferred field.
+Preparatory work for key-validity features.
diff --git a/changelog.d/5347.misc b/changelog.d/5347.misc
index 436245fb11..8336bc55dc 100644
--- a/changelog.d/5347.misc
+++ b/changelog.d/5347.misc
@@ -1,2 +1 @@
-Various improvements to debug logging.
-
+Preparatory work for key-validity features.
diff --git a/changelog.d/5352.bugfix b/changelog.d/5352.bugfix
new file mode 100644
index 0000000000..2ffefe5a68
--- /dev/null
+++ b/changelog.d/5352.bugfix
@@ -0,0 +1 @@
+Fix room stats and presence background updates to correctly handle missing events.
diff --git a/changelog.d/5356.misc b/changelog.d/5356.misc
new file mode 100644
index 0000000000..8336bc55dc
--- /dev/null
+++ b/changelog.d/5356.misc
@@ -0,0 +1 @@
+Preparatory work for key-validity features.
diff --git a/changelog.d/5357.doc b/changelog.d/5357.doc
new file mode 100644
index 0000000000..27cba49641
--- /dev/null
+++ b/changelog.d/5357.doc
@@ -0,0 +1 @@
+Fix notes about ACME in the MSC1711 faq.
diff --git a/changelog.d/5360.feature b/changelog.d/5360.feature
new file mode 100644
index 0000000000..01fbb3b06d
--- /dev/null
+++ b/changelog.d/5360.feature
@@ -0,0 +1 @@
+Update /_matrix/client/versions to reference support for r0.5.0. 
diff --git a/changelog.d/5362.bugfix b/changelog.d/5362.bugfix
new file mode 100644
index 0000000000..1c8b19182c
--- /dev/null
+++ b/changelog.d/5362.bugfix
@@ -0,0 +1 @@
+Fix `federation_custom_ca_list` configuration option.
diff --git a/docs/MSC1711_certificates_FAQ.md b/docs/MSC1711_certificates_FAQ.md
index ebfb20f5c8..37f7f669c9 100644
--- a/docs/MSC1711_certificates_FAQ.md
+++ b/docs/MSC1711_certificates_FAQ.md
@@ -145,12 +145,11 @@ You can do this with a `.well-known` file as follows:
  1. Keep the SRV record in place - it is needed for backwards compatibility
     with Synapse 0.34 and earlier.
 
- 2. Give synapse a certificate corresponding to the target domain
-    (`customer.example.net` in the above example). Currently Synapse's ACME
-    support [does not support
-    this](https://github.com/matrix-org/synapse/issues/4552), so you will have
-    to acquire a certificate yourself and give it to Synapse via
-    `tls_certificate_path` and `tls_private_key_path`.
+  2. Give Synapse a certificate corresponding to the target domain
+    (`customer.example.net` in the above example). You can either use Synapse's 
+    built-in [ACME support](./ACME.md) for this (via the `domain` parameter in 
+    the `acme` section), or acquire a certificate yourself and give it to 
+    Synapse via `tls_certificate_path` and `tls_private_key_path`.
 
  3. Restart Synapse to ensure the new certificate is loaded.
 
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index d10a355d68..2f37e71601 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -261,6 +261,22 @@ listeners:
 
 # Monthly Active User Blocking
 #
+# Used in cases where the admin or server owner wants to limit to the
+# number of monthly active users.
+#
+# 'limit_usage_by_mau' disables/enables monthly active user blocking. When
+# anabled and a limit is reached the server returns a 'ResourceLimitError'
+# with error type Codes.RESOURCE_LIMIT_EXCEEDED
+#
+# 'max_mau_value' is the hard limit of monthly active users above which
+# the server will start blocking user actions.
+#
+# 'mau_trial_days' is a means to add a grace period for active users. It
+# means that users must be active for this number of days before they
+# can be considered active and guards against the case where lots of users
+# sign up in a short space of time never to return after their initial
+# session.
+#
 #limit_usage_by_mau: False
 #max_mau_value: 50
 #mau_trial_days: 2
diff --git a/synapse/config/server.py b/synapse/config/server.py
index e763e19e15..334921d421 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -585,6 +585,22 @@ class ServerConfig(Config):
 
         # Monthly Active User Blocking
         #
+        # Used in cases where the admin or server owner wants to limit to the
+        # number of monthly active users.
+        #
+        # 'limit_usage_by_mau' disables/enables monthly active user blocking. When
+        # anabled and a limit is reached the server returns a 'ResourceLimitError'
+        # with error type Codes.RESOURCE_LIMIT_EXCEEDED
+        #
+        # 'max_mau_value' is the hard limit of monthly active users above which
+        # the server will start blocking user actions.
+        #
+        # 'mau_trial_days' is a means to add a grace period for active users. It
+        # means that users must be active for this number of days before they
+        # can be considered active and guards against the case where lots of users
+        # sign up in a short space of time never to return after their initial
+        # session.
+        #
         #limit_usage_by_mau: False
         #max_mau_value: 50
         #mau_trial_days: 2
diff --git a/synapse/config/tls.py b/synapse/config/tls.py
index 43712b8213..658f9dd361 100644
--- a/synapse/config/tls.py
+++ b/synapse/config/tls.py
@@ -107,7 +107,7 @@ class TlsConfig(Config):
             certs = []
             for ca_file in custom_ca_list:
                 logger.debug("Reading custom CA certificate file: %s", ca_file)
-                content = self.read_file(ca_file)
+                content = self.read_file(ca_file, "federation_custom_ca_list")
 
                 # Parse the CA certificates
                 try:
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index cf4fad7de0..ac5ca79143 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -35,6 +35,7 @@ from synapse.api.errors import (
     CodeMessageException,
     FederationDeniedError,
     FederationError,
+    RequestSendFailed,
     StoreError,
     SynapseError,
 )
@@ -2027,9 +2028,21 @@ class FederationHandler(BaseHandler):
         """
         room_version = yield self.store.get_room_version(event.room_id)
 
-        yield self._update_auth_events_and_context_for_auth(
-            origin, event, context, auth_events
-        )
+        try:
+            yield self._update_auth_events_and_context_for_auth(
+                origin, event, context, auth_events
+            )
+        except Exception:
+            # We don't really mind if the above fails, so lets not fail
+            # processing if it does. However, it really shouldn't fail so
+            # let's still log as an exception since we'll still want to fix
+            # any bugs.
+            logger.exception(
+                "Failed to double check auth events for %s with remote. "
+                "Ignoring failure and continuing processing of event.",
+                event.event_id,
+            )
+
         try:
             self.auth.check(room_version, event, auth_events=auth_events)
         except AuthError as e:
@@ -2042,6 +2055,15 @@ class FederationHandler(BaseHandler):
     ):
         """Helper for do_auth. See there for docs.
 
+        Checks whether a given event has the expected auth events. If it
+        doesn't then we talk to the remote server to compare state to see if
+        we can come to a consensus (e.g. if one server missed some valid
+        state).
+
+        This attempts to resovle any potential divergence of state between
+        servers, but is not essential and so failures should not block further
+        processing of the event.
+
         Args:
             origin (str):
             event (synapse.events.EventBase):
@@ -2088,9 +2110,15 @@ class FederationHandler(BaseHandler):
                 missing_auth,
             )
             try:
-                remote_auth_chain = yield self.federation_client.get_event_auth(
-                    origin, event.room_id, event.event_id
-                )
+                try:
+                    remote_auth_chain = yield self.federation_client.get_event_auth(
+                        origin, event.room_id, event.event_id
+                    )
+                except RequestSendFailed as e:
+                    # The other side isn't around or doesn't implement the
+                    # endpoint, so lets just bail out.
+                    logger.info("Failed to get event auth from remote: %s", e)
+                    return
 
                 seen_remotes = yield self.store.have_seen_events(
                     [e.event_id for e in remote_auth_chain]
@@ -2236,12 +2264,18 @@ class FederationHandler(BaseHandler):
 
         try:
             # 2. Get remote difference.
-            result = yield self.federation_client.query_auth(
-                origin,
-                event.room_id,
-                event.event_id,
-                local_auth_chain,
-            )
+            try:
+                result = yield self.federation_client.query_auth(
+                    origin,
+                    event.room_id,
+                    event.event_id,
+                    local_auth_chain,
+                )
+            except RequestSendFailed as e:
+                # The other side isn't around or doesn't implement the
+                # endpoint, so lets just bail out.
+                logger.info("Failed to query auth from remote: %s", e)
+                return
 
             seen_remotes = yield self.store.have_seen_events(
                 [e.event_id for e in result["auth_chain"]]
diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 6209858bbb..e49c8203ef 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -828,14 +828,17 @@ class PresenceHandler(object):
                 # joins.
                 continue
 
-            event = yield self.store.get_event(event_id)
-            if event.content.get("membership") != Membership.JOIN:
+            event = yield self.store.get_event(event_id, allow_none=True)
+            if not event or event.content.get("membership") != Membership.JOIN:
                 # We only care about joins
                 continue
 
             if prev_event_id:
-                prev_event = yield self.store.get_event(prev_event_id)
-                if prev_event.content.get("membership") == Membership.JOIN:
+                prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
+                if (
+                    prev_event
+                    and prev_event.content.get("membership") == Membership.JOIN
+                ):
                     # Ignore changes to join events.
                     continue
 
diff --git a/synapse/handlers/stats.py b/synapse/handlers/stats.py
index 0e92b405ba..7ad16c8566 100644
--- a/synapse/handlers/stats.py
+++ b/synapse/handlers/stats.py
@@ -115,6 +115,7 @@ class StatsHandler(StateDeltasHandler):
             event_id = delta["event_id"]
             stream_id = delta["stream_id"]
             prev_event_id = delta["prev_event_id"]
+            stream_pos = delta["stream_id"]
 
             logger.debug("Handling: %r %r, %s", typ, state_key, event_id)
 
@@ -136,10 +137,15 @@ class StatsHandler(StateDeltasHandler):
             event_content = {}
 
             if event_id is not None:
-                event_content = (yield self.store.get_event(event_id)).content or {}
+                event = yield self.store.get_event(event_id, allow_none=True)
+                if event:
+                    event_content = event.content or {}
+
+            # We use stream_pos here rather than fetch by event_id as event_id
+            # may be None
+            now = yield self.store.get_received_ts_by_stream_pos(stream_pos)
 
             # quantise time to the nearest bucket
-            now = yield self.store.get_received_ts(event_id)
             now = (now // 1000 // self.stats_bucket_size) * self.stats_bucket_size
 
             if typ == EventTypes.Member:
@@ -149,9 +155,11 @@ class StatsHandler(StateDeltasHandler):
                 # compare them.
                 prev_event_content = {}
                 if prev_event_id is not None:
-                    prev_event_content = (
-                        yield self.store.get_event(prev_event_id)
-                    ).content
+                    prev_event = yield self.store.get_event(
+                        prev_event_id, allow_none=True,
+                    )
+                    if prev_event:
+                        prev_event_content = prev_event.content
 
                 membership = event_content.get("membership", Membership.LEAVE)
                 prev_membership = prev_event_content.get("membership", Membership.LEAVE)
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index 27e7cbf3cc..babbf6a23c 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -39,6 +39,7 @@ class VersionsRestServlet(RestServlet):
                 "r0.2.0",
                 "r0.3.0",
                 "r0.4.0",
+                "r0.5.0",
             ],
             # as per MSC1497:
             "unstable_features": {
diff --git a/synapse/storage/events_worker.py b/synapse/storage/events_worker.py
index 1782428048..cc7df5cf14 100644
--- a/synapse/storage/events_worker.py
+++ b/synapse/storage/events_worker.py
@@ -78,6 +78,43 @@ class EventsWorkerStore(SQLBaseStore):
             desc="get_received_ts",
         )
 
+    def get_received_ts_by_stream_pos(self, stream_ordering):
+        """Given a stream ordering get an approximate timestamp of when it
+        happened.
+
+        This is done by simply taking the received ts of the first event that
+        has a stream ordering greater than or equal to the given stream pos.
+        If none exists returns the current time, on the assumption that it must
+        have happened recently.
+
+        Args:
+            stream_ordering (int)
+
+        Returns:
+            Deferred[int]
+        """
+
+        def _get_approximate_received_ts_txn(txn):
+            sql = """
+                SELECT received_ts FROM events
+                WHERE stream_ordering >= ?
+                LIMIT 1
+            """
+
+            txn.execute(sql, (stream_ordering,))
+            row = txn.fetchone()
+            if row and row[0]:
+                ts = row[0]
+            else:
+                ts = self.clock.time_msec()
+
+            return ts
+
+        return self.runInteraction(
+            "get_approximate_received_ts",
+            _get_approximate_received_ts_txn,
+        )
+
     @defer.inlineCallbacks
     def get_event(
         self,
diff --git a/tests/handlers/test_stats.py b/tests/handlers/test_stats.py
index 249aba3d59..2710c991cf 100644
--- a/tests/handlers/test_stats.py
+++ b/tests/handlers/test_stats.py
@@ -204,7 +204,7 @@ class StatsRoomTests(unittest.HomeserverTestCase):
             "a2": {"membership": "not a real thing"},
         }
 
-        def get_event(event_id):
+        def get_event(event_id, allow_none=True):
             m = Mock()
             m.content = events[event_id]
             d = defer.Deferred()
@@ -224,7 +224,7 @@ class StatsRoomTests(unittest.HomeserverTestCase):
                 "room_id": "room",
                 "event_id": "a1",
                 "prev_event_id": "a2",
-                "stream_id": "bleb",
+                "stream_id": 60,
             }
         ]
 
@@ -241,7 +241,7 @@ class StatsRoomTests(unittest.HomeserverTestCase):
                 "room_id": "room",
                 "event_id": "a2",
                 "prev_event_id": "a1",
-                "stream_id": "bleb",
+                "stream_id": 100,
             }
         ]
 
@@ -249,3 +249,59 @@ class StatsRoomTests(unittest.HomeserverTestCase):
         self.assertEqual(
             f.value.args[0], "'not a real thing' is not a valid membership"
         )
+
+    def test_redacted_prev_event(self):
+        """
+        If the prev_event does not exist, then it is assumed to be a LEAVE.
+        """
+        u1 = self.register_user("u1", "pass")
+        u1_token = self.login("u1", "pass")
+
+        room_1 = self.helper.create_room_as(u1, tok=u1_token)
+
+        # Do the initial population of the user directory via the background update
+        self._add_background_updates()
+
+        while not self.get_success(self.store.has_completed_background_updates()):
+            self.get_success(self.store.do_next_background_update(100), by=0.1)
+
+        events = {
+            "a1": None,
+            "a2": {"membership": Membership.JOIN},
+        }
+
+        def get_event(event_id, allow_none=True):
+            if events.get(event_id):
+                m = Mock()
+                m.content = events[event_id]
+            else:
+                m = None
+            d = defer.Deferred()
+            self.reactor.callLater(0.0, d.callback, m)
+            return d
+
+        def get_received_ts(event_id):
+            return defer.succeed(1)
+
+        self.store.get_received_ts = get_received_ts
+        self.store.get_event = get_event
+
+        deltas = [
+            {
+                "type": EventTypes.Member,
+                "state_key": "some_user:test",
+                "room_id": room_1,
+                "event_id": "a2",
+                "prev_event_id": "a1",
+                "stream_id": 100,
+            }
+        ]
+
+        # Handle our fake deltas, which has a user going from LEAVE -> JOIN.
+        self.get_success(self.handler._handle_deltas(deltas))
+
+        # One delta, with two joined members -- the room creator, and our fake
+        # user.
+        r = self.get_success(self.store.get_deltas_for_room(room_1, 0))
+        self.assertEqual(len(r), 1)
+        self.assertEqual(r[0]["joined_members"], 2)