diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index e4343e8..91ab085 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -383,6 +383,14 @@
 
     .. note::
 
+        The :class:`catch_warnings` manager works by replacing and
+        then later restoring the module's
+        :func:`showwarning` function and internal list of filter
+        specifications.  This means the context manager is modifying
+        global state and therefore is not thread-safe.
+
+    .. note::
+
         In Python 3.0, the arguments to the constructor for
         :class:`catch_warnings` are keyword-only arguments.