diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 69751a82df..5d9f459411 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -93,6 +93,8 @@ LEGACY_API_DIRS = (
   'voice_engine/include',
 )
 
+# NOTE: The set of directories in API_DIRS should be the same as those
+# listed in the table in native-api.md.
 API_DIRS = NATIVE_API_DIRS[:] + LEGACY_API_DIRS[:]
 
 # TARGET_RE matches a GN target, and extracts the target name and the contents.
diff --git a/README.md b/README.md
index 70d7c5f3e5..2a24595476 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,9 @@ amongst others.
 See http://www.webrtc.org/native-code/development for instructions on how to get
 started developing with the native code.
 
+[Authoritative list](native-api.md) of directories that contain the
+native API header files.
+
 ### More info
 
  * Official web site: http://www.webrtc.org
diff --git a/native-api.md b/native-api.md
new file mode 100644
index 0000000000..b0ebe2762f
--- /dev/null
+++ b/native-api.md
@@ -0,0 +1,67 @@
+# API header files
+
+As a user of the WebRTC library, you may use headers and build files
+in the following directories:
+
+API directory | Including subdirectories?
+--------------|-------------------------
+`api`         | Yes
+
+For now, you may also use headers and build files in the following
+legacy API directories—but see the
+[disclaimer](#legacy-disclaimer) below.
+
+Legacy API directory                       | Including subdirectories?
+-------------------------------------------|--------------------------
+`common_audio/include`                     | No
+`media`                                    | No
+`modules/audio_coding/include`             | No
+`modules/audio_device/include`             | No
+`modules/audio_processing/include`         | No
+`modules/bitrate_controller/include`       | No
+`modules/congestion_controller/include`    | No
+`modules/include`                          | No
+`modules/remote_bitrate_estimator/include` | No
+`modules/rtp_rtcp/include`                 | No
+`modules/rtp_rtcp/source`                  | No
+`modules/utility/include`                  | No
+`modules/video_coding/codecs/h264/include` | No
+`modules/video_coding/codecs/i420/include` | No
+`modules/video_coding/codecs/vp8/include`  | No
+`modules/video_coding/codecs/vp9/include`  | No
+`modules/video_coding/include`             | No
+`pc`                                       | No
+`rtc_base`                                 | No
+`system_wrappers/include`                  | No
+`voice_engine/include`                     | No
+
+While the files, types, functions, macros, build targets, etc. in the
+API and legacy API directories will sometimes undergo incompatible
+changes, such changes will be announced in advance to
+[discuss-webrtc@googlegroups.com][discuss-webrtc], and a migration
+path will be provided.
+
+[discuss-webrtc]: https://groups.google.com/forum/#!forum/discuss-webrtc
+
+In the directories not listed in the tables above, incompatible
+changes may happen at any time, and are not announced.
+
+## <a name="legacy-disclaimer"></a>The legacy API directories contain some things you shouldn&rsquo;t use
+
+The legacy API directories, in addition to things that genuinely
+should be part of the API, also contain things that should *not* be
+part of the API. We are in the process of moving the good stuff to the
+`api` directory tree, and will remove directories from the legacy list
+once they no longer contain anything that should be in the API.
+
+In other words, if you find things in the legacy API directories that
+don&rsquo;t seem like they belong in the WebRTC native API,
+don&rsquo;t grow too attached to them.
+
+## All these worlds are yours&mdash;except Europa
+
+In the API headers, or in files included by the API headers, there are
+types, functions, namespaces, etc. that have `impl` or `internal` in
+their names (in various styles, such as `CamelCaseImpl`,
+`snake_case_impl`). They are not part of the API, and may change
+incompatibly at any time; do not use them.