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+# Testing
+
+We verify and test Marionette in a couple of different ways, using
+a combination of unit tests and functional tests.  There are three
+distinct components that we test:
+
+- the Marionette **server**, using a combination of xpcshell
+unit tests and functional tests written in Python spread across
+Marionette- and WPT tests;
+
+- the Python **client** is tested with the same body of functional
+Marionette tests;
+
+- and the **harness** that backs the Marionette, or `Mn` job on
+try, tests is verified using separate mock-styled unit tests.
+
+All these tests can be run by using [mach].
+
+[mach]: https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/mach
+
+## xpcshell unit tests
+
+Marionette has a set of [xpcshell] unit tests located in
+_remote/marionette/test/xpcshell.  These can be run this way:
+
+    % ./mach test remote/marionette/test/unit
+
+Because tests are run in parallel and xpcshell itself is quite
+chatty, it can sometimes be useful to run the tests sequentially:
+
+    % ./mach test --sequential remote/marionette/test/xpcshell/test_error.js
+
+These unit tests run as part of the `X` jobs on Treeherder.
+
+[xpcshell]: https://developer.mozilla.org/en-US/docs/Mozilla/QA/Writing_xpcshell-based_unit_tests
+
+## Marionette functional tests
+
+We also have a set of [functional tests] that make use of the Marionette
+Python client.  These start a Firefox process and tests the Marionette
+protocol input and output, and will appear as `Mn` on Treeherder.
+The following command will run all tests locally:
+
+    % ./mach marionette-test
+
+But you can also run individual tests:
+
+    % ./mach marionette-test testing/marionette/harness/marionette_harness/tests/unit/test_navigation.py
+
+In case you want to run the tests with another binary like [Firefox Nightly]:
+
+    % ./mach marionette-test --binary /path/to/nightly/firefox TEST
+
+When working on Marionette it is often useful to surface the stdout
+from Gecko, which can be achieved using the `--gecko-log` option.
+See [Debugging](Debugging.md) for usage instructions, but the gist is that
+you can redirect all Gecko output to stdout:
+
+    % ./mach marionette-test --gecko-log - TEST
+
+Our functional integration tests pop up Firefox windows sporadically,
+and a helpful tip is to suppress the window can be to use Firefox’
+headless mode:
+
+    % ./mach marionette-test -z TEST
+
+`-z` is an alias for the `--headless` flag and equivalent to setting
+the `MOZ_HEADLESS` output variable.  In addition to `MOZ_HEADLESS`
+there is also `MOZ_HEADLESS_WIDTH` and `MOZ_HEADLESS_HEIGHT` for
+controlling the dimensions of the no-op virtual display.  This is
+similar to using Xvfb(1) which you may know from the X windowing system,
+but has the additional benefit of also working on macOS and Windows.
+
+[functional tests]: PythonTests.md
+[Firefox Nightly]: https://nightly.mozilla.org/
+
+### Android
+
+Prerequisites:
+
+- You have [built Fennec](https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Build_Instructions/Simple_Firefox_for_Android_build).
+- You can run an Android [emulator](https://wiki.mozilla.org/Mobile/Fennec/Android/Testing#Running_tests_on_the_Android_emulator),
+    which means you have the AVD you need.
+
+When running tests on Fennec, you can have Marionette runner take care of
+starting Fennec and an emulator, as shown below.
+
+    % ./mach marionette-test --emulator --app fennec
+      --avd-home /path/to/.mozbuild/android-device/avd
+      --emulator-binary /path/to/.mozbuild/android-sdk/emulator/emulator
+      --avd=mozemulator-x86
+
+For Fennec tests, if the appropriate `emulator` command is in your `PATH`, you may omit the `--emulator-binary` argument.  See `./mach marionette-test -h`
+for additional options.
+
+Alternately, you can start an emulator yourself and have the Marionette runner
+start Fennec for you:
+
+    % ./mach marionette-test --emulator --app='fennec' --address=127.0.0.1:2828
+
+To connect to an already-running Fennec in an emulator or on a device,
+you will need to have it started with the `-marionette` command line argument,
+or by setting the environment variable `MOZ_MARIONETTE=1` for the process.
+
+Make sure port 2828 is forwarded:
+
+    % adb forward tcp:2828 tcp:2828
+
+If Fennec is already started:
+
+    % ./mach marionette-test --app='fennec' --address=127.0.0.1:2828
+
+If Fennec is not already started on the emulator/device, add the `--emulator`
+option. Marionette Test Runner will take care of forwarding the port and
+starting Fennec with the correct prefs. (You may need to run
+`adb forward --remove-all` to allow the runner to start.)
+
+    % ./mach marionette-test --emulator --app='fennec' --address=127.0.0.1:2828 --startup-timeout=300
+
+If you need to troubleshoot the Marionette connection, the most basic check is
+to start Fennec with `-marionette` or the environment variable `MOZ_MARIONETTE=1`,
+make sure that the port 2828 is forwarded, and then see if you get any response from
+Marionette when you connect manually:
+
+    % telnet 127.0.0.1:2828
+
+You should see output like `{"applicationType":"gecko","marionetteProtocol":3}`
+
+[geckodriver]: /testing/geckodriver/index.rst
+
+## WPT functional tests
+
+Marionette is also indirectly tested through [geckodriver] with WPT
+(`Wd` on Treeherder).  To run them:
+
+    % ./mach wpt testing/web-platform/tests/webdriver
+
+WPT tests conformance to the [WebDriver] standard and uses
+[geckodriver].  Together with the Marionette remote protocol in
+Gecko, they make up Mozilla’s WebDriver implementation.
+
+This command supports a `--webdriver-arg='-vv'` argument that
+enables more detailed logging, as well as `--jsdebugger` for opening
+the Browser Toolbox.
+
+A particularly useful trick is to combine this with the headless
+mode for Firefox:
+
+    % ./mach wpt --webdriver-arg='-vv' --headless testing/web-platform/tests/webdriver
+
+[WebDriver]: https://w3c.github.io/webdriver/
+
+## Harness tests
+
+The Marionette harness Python package has a set of mock-styled unit
+tests that uses the [pytest] framework.  The following command will
+run all tests:
+
+    % ./mach python-test testing/marionette
+
+To run a specific test specify the full path to the module:
+
+    % ./mach python-test testing/marionette/harness/marionette_harness/tests/harness_unit/test_serve.py
+
+[pytest]: https://docs.pytest.org/en/latest/
+
+## One-click loaners
+
+Additionally, for debugging hard-to-reproduce test failures in CI,
+one-click loaners from [Taskcluster](Taskcluster.md) can be particularly useful.
+
+## Out-of-tree testing
+
+All the above examples show tests running _in-tree_, with a local
+checkout of _central_ and a local build of Firefox.  It is also
+possibly to run the Marionette tests _without_ a local build and
+with a downloaded test archive from [Taskcluster](Taskcluster.md)
+
+If you want to run tests from a downloaded test archive, you will
+need to download the `target.common.tests.tar.gz` artifact attached to
+Treeherder [build jobs] `B` for your system.  Extract the archive
+and set up the Python Marionette client and harness by executing
+the following command in a virtual environment:
+
+    % pip install -r config/marionette_requirements.txt
+
+The tests can then be found under
+_marionette/tests/testing/marionette/harness/marionette_harness/tests_
+and can be executed with the command `marionette`.  It supports
+the same options as described above for `mach`.
+
+[build jobs]: https://treeherder.mozilla.org/#/jobs?repo=mozilla-central&filter-searchStr=build