diff --git a/specification/server_server_api.rst b/specification/server_server_api.rst
index 352bc48e..01c3b65b 100644
--- a/specification/server_server_api.rst
+++ b/specification/server_server_api.rst
@@ -358,6 +358,7 @@ be inserted. The types of state events that affect authorization are:
 - ``m.room.member``
 - ``m.room.join_rules``
 - ``m.room.power_levels``
+- ``m.room.third_party_invite``
 
 Servers should not create new events that reference unauthorized events.
 However, any event that does reference an unauthorized event is not itself
@@ -412,7 +413,33 @@ The rules are as follows:
 
    #. If ``membership`` is ``invite``:
 
-      i. If the ``sender``'s current membership state is not ``join``, reject.
+      i. If ``content`` has ``third_party_invite`` key:
+
+         #. If *target user* is banned, reject.
+
+         #. If ``content.third_party_invite`` does not have a
+            ``signed`` key, reject.
+
+         #. If ``signed`` does not have ``mxid`` and ``token`` keys, reject.
+
+         #. If ``mxid`` does not match ``state_key``, reject.
+
+         #. If there is no ``m.room.third_party_invite`` event in the
+            current room state with ``state_key`` matching ``token``, reject.
+
+         #. If ``sender`` does not match ``sender`` of the
+            ``m.room.third_party_invite``, reject.
+
+         #. If any signature in ``signed`` matches any public key in the
+            ``m.room.third_party_invite`` event, allow. The public keys are
+            in ``content`` of ``m.room.third_party_invite`` as:
+
+            #. A single public key in the ``public_key`` field.
+            #. A list of public keys in the ``public_keys`` field.
+
+        #. Otherwise, reject.
+
+      #. If the ``sender``'s current membership state is not ``join``, reject.
 
       #. If *target user*'s current membership state is ``join`` or ``ban``,
          reject.
@@ -452,6 +479,11 @@ The rules are as follows:
 
 #. If the ``sender``'s current membership state is not ``join``, reject.
 
+#. If type is ``m.room.third_party_invite``:
+
+   a. Allow if and only if ``sender``'s current power level is greater than
+      or equal to the *invite level*.
+
 #. If the event type's *required power level* is greater than the ``sender``'s power
    level, reject.
 
@@ -508,9 +540,6 @@ The rules are as follows:
     the kick *and* ban levels, *and* greater than the target user's power
     level.
 
-.. TODO-spec
-
-  I think there is some magic about 3pid invites too.
 
 Retrieving event authorization information
 ++++++++++++++++++++++++++++++++++++++++++