diff --git a/source/fundamentals/serialization/poco.txt b/source/fundamentals/serialization/poco.txt
index 8819e5b1..d8dcc294 100644
--- a/source/fundamentals/serialization/poco.txt
+++ b/source/fundamentals/serialization/poco.txt
@@ -256,6 +256,11 @@ The following code sample maps the ``Identifier`` property to the
    (for example, if your POCO includes properties named ``Id`` and ``_id``), 
    the driver throws a ``BsonSerializationException``.
 
+.. note:: Nested Document Ids
+
+   The ``_id`` field mapping logic described in this section only applies to the
+   root document and does not apply to nested documents.
+
 Omit Empty Fields
 ~~~~~~~~~~~~~~~~~