diff --git a/doc/cache.rst b/doc/cache.rst
index b9dbf2797..a39a1e76c 100644
--- a/doc/cache.rst
+++ b/doc/cache.rst
@@ -19,8 +19,18 @@ a lower version number is found the cache is recreated with the current
 version. If a higher version number is found the cache is ignored and left as
 is.
 
-Snapshots and Indexes
----------------------
+Snapshots, Data and Indexes
+---------------------------
 
 Snapshot, Data and Index files are cached in the sub-directories ``snapshots``,
 ``data`` and  ``index``, as read from the repository.
+
+Expiry
+------
+
+Whenever a cache directory for a repo is used, that directory's modification
+timestamp is updated to the current time. By looking at the modification
+timestamps of the repo cache directories it is easy to decide which directories
+are old and haven't been used in a long time. Those are probably stale and can
+be removed.
+
diff --git a/doc/manual_rest.rst b/doc/manual_rest.rst
index a3da3453e..be2e26805 100644
--- a/doc/manual_rest.rst
+++ b/doc/manual_rest.rst
@@ -280,3 +280,10 @@ entirely. In this case, all data is loaded from the repo.
 
 The cache is ephemeral: When a file cannot be read from the cache, it is loaded
 from the repository.
+
+Within the cache directory, there's a sub directory for each repository the
+cache was used with. Restic updates the timestamps of a repo directory each
+time it is used, so by looking at the timestamps of the sub directories of the
+cache directory it can decide which sub directories are old and probably not
+needed any more. You can either remove these directories manually, or run a
+restic command with the ``--cleanup-cache`` flag.