diff --git a/specification/server_server_api.rst b/specification/server_server_api.rst
index 3738ea05..f2b3204a 100644
--- a/specification/server_server_api.rst
+++ b/specification/server_server_api.rst
@@ -16,6 +16,10 @@
 Federation API
 ==============
 
+.. WARNING::
+  This API is unstable and will change without warning or discussion while
+  we work towards a r0 release (scheduled for August 2018).
+
 Matrix homeservers use the Federation APIs (also known as server-server APIs)
 to communicate with each other. Homeservers use these APIs to push messages to
 each other in real-time, to