--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -908,8 +908,42 @@ class MachDebug(object):
             if verbose:
                 print('config substitutions:')
                 for k in sorted(config.substs):
                     print('\t%s: %s' % (k, config.substs[k]))
 
                 print('config defines:')
                 for k in sorted(config.defines):
                     print('\t%s' % k)
+
+
+@CommandProvider
+class Documentation(MachCommandBase):
+    """Helps manage in-tree documentation."""
+
+    @Command('build-docs', category='build-dev',
+        description='Generate documentation for the tree.')
+    @CommandArgument('--format', default='html',
+        help='Documentation format to write.')
+    @CommandArgument('outdir', default='<DEFAULT>', nargs='?',
+        help='Where to write output.')
+    def build_docs(self, format=None, outdir=None):
+        self._activate_virtualenv()
+
+        self.virtualenv_manager.install_pip_package('mdn-sphinx-theme==0.3')
+
+        import sphinx
+
+        if outdir == '<DEFAULT>':
+            outdir = os.path.join(self.topobjdir, 'docs', format)
+
+        args = [
+            sys.argv[0],
+            '-b', format,
+            os.path.join(self.topsrcdir, 'build', 'docs'),
+            outdir,
+        ]
+
+        result = sphinx.main(args)
+
+        print('Docs written to %s.' % outdir)
+
+        return result