diff --git a/optimade.rst b/optimade.rst
index f1cfc94fc..86fc1ec1d 100644
--- a/optimade.rst
+++ b/optimade.rst
@@ -493,6 +493,10 @@ Every response SHOULD contain the following fields, and MUST contain at least :f
   - **data\_available**: an integer containing the total number of data resource objects available in the database for the endpoint.
   - **last\_id**: a string containing the last ID returned.
   - **response\_message**: response string from the server.
+  - **request\_delay**: a non-negative float giving time in seconds that the client is suggested to wait before issuing a subsequent request.
+
+  Implementation note: the functionality of this field overlaps to some degree with features provided by the HTTP error :http-error:`429 Too Many Requests` and the `Retry-After HTTP header <https://tools.ietf.org/html/rfc7231.html#section-7.1.3>`__. Implementations are suggested to provide consistent handling of request overload through both mechanisms.
+
   - **implementation**: a dictionary describing the server implementation, containing the OPTIONAL fields:
 
     - **name**: name of the implementation.