diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index 2d7d4f02..b5e28bac 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -55,6 +55,7 @@ phutil_register_library_map(array(
     'ArcanistLinterTestCase' => 'lint/linter/base/test',
     'ArcanistListWorkflow' => 'workflow/list',
     'ArcanistMarkCommittedWorkflow' => 'workflow/mark-committed',
+    'ArcanistMercurialAPI' => 'repository/api/mercurial',
     'ArcanistNoEffectException' => 'exception/usage/noeffect',
     'ArcanistNoEngineException' => 'exception/usage/noengine',
     'ArcanistNoLintLinter' => 'lint/linter/nolint',
@@ -118,6 +119,7 @@ phutil_register_library_map(array(
     'ArcanistLinterTestCase' => 'ArcanistPhutilTestCase',
     'ArcanistListWorkflow' => 'ArcanistBaseWorkflow',
     'ArcanistMarkCommittedWorkflow' => 'ArcanistBaseWorkflow',
+    'ArcanistMercurialAPI' => 'ArcanistRepositoryAPI',
     'ArcanistNoEffectException' => 'ArcanistUsageException',
     'ArcanistNoEngineException' => 'ArcanistUsageException',
     'ArcanistNoLintLinter' => 'ArcanistLinter',
diff --git a/src/parser/diff/ArcanistDiffParser.php b/src/parser/diff/ArcanistDiffParser.php
index cd57f107..f7fece8f 100644
--- a/src/parser/diff/ArcanistDiffParser.php
+++ b/src/parser/diff/ArcanistDiffParser.php
@@ -27,6 +27,7 @@ class ArcanistDiffParser {
   protected $text;
   protected $line;
   protected $isGit;
+  protected $isMercurial;
   protected $detectBinaryFiles = false;
 
   protected $changes = array();
@@ -209,6 +210,9 @@ class ArcanistDiffParser {
         '(?P<binary>Binary) files '.
           '(?P<old>.+)\s+\d{4}-\d{2}-\d{2} and '.
           '(?P<new>.+)\s+\d{4}-\d{2}-\d{2} differ.*',
+
+        // This is a normal Mercurial text change, probably from "hg diff".
+        '(?P<type>diff -r) (?P<hgrev>[a-f0-9]+) (?P<cur>.+)',
       );
 
       $ok = false;
@@ -274,6 +278,10 @@ class ArcanistDiffParser {
           $line = $this->nextLine();
           $this->parseChangeset($change);
           break;
+        case 'diff -r':
+          $this->setIsMercurial(true);
+          $this->parseIndexHunk($change);
+          break;
         default:
           $this->didFailParse("Unknown diff type.");
       }
@@ -432,8 +440,19 @@ class ArcanistDiffParser {
     return $this->isGit;
   }
 
+  public function setIsMercurial($is_mercurial) {
+    $this->isMercurial = $is_mercurial;
+    return $this;
+  }
+
+  public function getIsMercurial() {
+    return $this->isMercurial;
+  }
+
   protected function parseIndexHunk(ArcanistDiffChange $change) {
     $is_git = $this->getIsGit();
+    $is_mercurial = $this->getIsMercurial();
+    $is_svn = (!$is_git && !$is_mercurial);
 
     $line = $this->getLine();
     if ($is_git) {
@@ -532,19 +551,27 @@ class ArcanistDiffParser {
     }
 
     $line = $this->getLine();
-    $ok = preg_match('/^=+$/', $line) ||
-          ($is_git && preg_match('/^index .*$/', $line));
-    if (!$ok) {
-      if ($is_git) {
-        $this->didFailParse(
-          "Expected 'index af23f...a98bc' header line.");
+
+    if ($is_svn) {
+      $ok = preg_match('/^=+$/', $line);
+      if (!$ok) {
+        $this->didFailParse("Expected '=======================' divider line.");
       } else {
-        $this->didFailParse(
-          "Expected '==========================' divider line.");
+        // Adding an empty file in SVN can produce an empty line here.
+        $line = $this->nextNonemptyLine();
+      }
+    } else if ($is_git) {
+      $ok = preg_match('/^index .*$/', $line);
+      if (!$ok) {
+        // TODO: "hg diff -g" diffs ("mercurial git-style diffs") do not include
+        // this line, so we can't parse them if we fail on it. Maybe introduce
+        // a flag saying "parse this diff using relaxed git-style diff rules"?
+
+        // $this->didFailParse("Expected 'index af23f...a98bc' header line.");
+      } else {
+        $line = $this->nextLine();
       }
     }
-    // Adding an empty file in SVN can produce an empty line here.
-    $line = $this->nextNonemptyLine();
 
     // If there are files with only whitespace changes and -b or -w are
     // supplied as command-line flags to `diff', svn and git both produce
@@ -596,14 +623,23 @@ class ArcanistDiffParser {
   protected function parseHunkTarget() {
     $line = $this->getLine();
     $matches = null;
+
+    $remainder = '(?:\s*\(.*\))?';
+    if ($this->getIsMercurial()) {
+      // Something like "Fri Aug 26 01:20:50 2005 -0700", don't bother trying
+      // to parse it.
+      $remainder = '\t.*';
+    }
+
     $ok = preg_match(
-      '@^[-+]{3} (?:[ab]/)?(?P<path>.*?)(?:\s*\(.*\))?$@',
+      '@^[-+]{3} (?:[ab]/)?(?P<path>.*?)'.$remainder.'$@',
       $line,
       $matches);
     if (!$ok) {
       $this->didFailParse(
         "Expected hunk target '+++ path/to/file.ext (revision N)'.");
     }
+
     $this->nextLine();
     return $matches['path'];
   }
diff --git a/src/repository/api/base/ArcanistRepositoryAPI.php b/src/repository/api/base/ArcanistRepositoryAPI.php
index 30df621a..1acdd39f 100644
--- a/src/repository/api/base/ArcanistRepositoryAPI.php
+++ b/src/repository/api/base/ArcanistRepositoryAPI.php
@@ -65,11 +65,21 @@ abstract class ArcanistRepositoryAPI {
         "any parent directory. Create an '.arcconfig' file to configure arc.");
     }
 
-    if (@file_exists($root.'/.svn')) {
-      phutil_require_module('arcanist', 'repository/api/subversion');
-      return new ArcanistSubversionAPI($root);
+    if (Filesystem::pathExists($root.'/.svn')) {
+      return newv('ArcanistSubversionAPI', array($root));
     }
 
+    if (Filesystem::pathExists($root.'/.hg')) {
+      // TODO: Stabilize and remove.
+      file_put_contents(
+        'php://stderr',
+        phutil_console_format(
+          "**WARNING:** Mercurial support is largely imaginary right now.\n"));
+
+      return newv('ArcanistMercurialAPI', array($root));
+    }
+
+
     $git_root = self::discoverGitBaseDirectory($root);
     if ($git_root) {
       if (!Filesystem::pathsAreEquivalent($root, $git_root)) {
@@ -77,16 +87,16 @@ abstract class ArcanistRepositoryAPI {
           "'.arcconfig' file is located at '{$root}', but working copy root ".
           "is '{$git_root}'. Move '.arcconfig' file to the working copy root.");
       }
-      phutil_require_module('arcanist', 'repository/api/git');
-      return new ArcanistGitAPI($root);
+
+      return newv('ArcanistGitAPI', array($root));
     }
 
     throw new ArcanistUsageException(
       "The current working directory is not part of a working copy for a ".
-      "supported version control system (svn or git).");
+      "supported version control system (svn, git or mercurial).");
   }
 
-  protected function __construct($path) {
+  public function __construct($path) {
     $this->path = $path;
   }
 
diff --git a/src/repository/api/base/__init__.php b/src/repository/api/base/__init__.php
index ad7f82cd..f2fcc047 100644
--- a/src/repository/api/base/__init__.php
+++ b/src/repository/api/base/__init__.php
@@ -8,8 +8,10 @@
 
 phutil_require_module('arcanist', 'exception/usage');
 
+phutil_require_module('phutil', 'console');
 phutil_require_module('phutil', 'filesystem');
 phutil_require_module('phutil', 'future/exec');
+phutil_require_module('phutil', 'utils');
 
 
 phutil_require_source('ArcanistRepositoryAPI.php');
diff --git a/src/repository/api/git/ArcanistGitAPI.php b/src/repository/api/git/ArcanistGitAPI.php
index c6234cb1..520aad78 100644
--- a/src/repository/api/git/ArcanistGitAPI.php
+++ b/src/repository/api/git/ArcanistGitAPI.php
@@ -97,7 +97,6 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
   }
 
   public function getRawDiffText($path) {
-    $relative_commit = $this->getRelativeCommit();
     $options = $this->getDiffFullOptions();
     list($stdout) = execx(
       "(cd %s; git diff {$options} %s -- %s)",
diff --git a/src/repository/api/mercurial/ArcanistMercurialAPI.php b/src/repository/api/mercurial/ArcanistMercurialAPI.php
new file mode 100644
index 00000000..16bcdffa
--- /dev/null
+++ b/src/repository/api/mercurial/ArcanistMercurialAPI.php
@@ -0,0 +1,142 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Interfaces with the Mercurial working copies.
+ *
+ * @group workingcopy
+ */
+class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
+
+  private $status;
+  private $base;
+
+  public function getSourceControlSystemName() {
+    return 'hg';
+  }
+
+  public function getSourceControlBaseRevision() {
+    list($stdout) = execx(
+      '(cd %s && hg id -ir %s)',
+      $this->getPath(),
+      $this->getRelativeCommit());
+    return $stdout;
+  }
+
+  public function getSourceControlPath() {
+    return '/';
+  }
+
+  public function getBranchName() {
+    // TODO: I have nearly no idea how hg local branches work.
+    list($stdout) = execx(
+      '(cd %s && hg branch)',
+      $this->getPath());
+    return $stdout;
+  }
+
+  public function getRelativeCommit() {
+    // TODO: This is hardcoded.
+    return 'tip~1';
+  }
+
+  public function getBlame($path) {
+    list($stdout) = execx(
+      '(cd %s && hg blame -u -v -c --rev %s -- %s)',
+      $this->getPath(),
+      $this->getRelativeCommit(),
+      $path);
+
+    $blame = array();
+    foreach (explode("\n", trim($stdout)) as $line) {
+      if (!strlen($line)) {
+        continue;
+      }
+
+      $matches = null;
+      $ok = preg_match('^/\s*([^:]+?) [a-f0-9]{12}: (.*)$/', $line, $matches);
+
+      if (!$ok) {
+        throw new Exception("Unable to parse Mercurial blame line: {$line}");
+      }
+
+      $revision = $matches[2];
+      $author = trim($matches[1]);
+      $blame[] = array($author, $revision);
+    }
+
+    return $blame;
+  }
+
+  public function getWorkingCopyStatus() {
+
+    // TODO: This is critical and not yet implemented.
+
+    return array();
+  }
+
+  private function getDiffOptions() {
+    $options = array(
+      '-g',
+      '-U'.$this->getDiffLinesOfContext(),
+    );
+    return implode(' ', $options);
+  }
+
+  public function getRawDiffText($path) {
+    $options = $this->getDiffOptions();
+
+    list($stdout) = execx(
+      '(cd %s && hg diff %C --rev %s --rev tip -- %s)',
+      $this->getPath(),
+      $options,
+      $this->getRelativeCommit(),
+      $path);
+
+    return $stdout;
+  }
+
+  public function getFullMercurialDiff() {
+    $options = $this->getDiffOptions();
+
+    list($stdout) = execx(
+      '(cd %s && hg diff %C --rev %s --rev tip --)',
+      $this->getPath(),
+      $options,
+      $this->getRelativeCommit());
+
+    return $stdout;
+  }
+
+  public function getOriginalFileData($path) {
+    return $this->getFileDataAtRevision($path, $this->getRelativeCommit());
+  }
+
+  public function getCurrentFileData($path) {
+    return $this->getFileDataAtRevision($path, 'tip');
+  }
+
+  private function getFileDataAtRevision($path, $revision) {
+    list($stdout) = execx(
+      '(cd %s && hg cat --rev %s -- %s)',
+      $this->getPath(),
+      $path);
+    return $stdout;
+  }
+
+}
diff --git a/src/repository/api/mercurial/__init__.php b/src/repository/api/mercurial/__init__.php
new file mode 100644
index 00000000..480bf00d
--- /dev/null
+++ b/src/repository/api/mercurial/__init__.php
@@ -0,0 +1,14 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('arcanist', 'repository/api/base');
+
+phutil_require_module('phutil', 'future/exec');
+
+
+phutil_require_source('ArcanistMercurialAPI.php');
diff --git a/src/workflow/diff/ArcanistDiffWorkflow.php b/src/workflow/diff/ArcanistDiffWorkflow.php
index bf365d9f..0a8c4cde 100644
--- a/src/workflow/diff/ArcanistDiffWorkflow.php
+++ b/src/workflow/diff/ArcanistDiffWorkflow.php
@@ -280,8 +280,12 @@ EOTEXT
       if ($info['uuid']) {
         $repo_uuid = $info['uuid'];
       }
-    } else {
+    } else if ($repository_api instanceof ArcanistSubversionAPI) {
       $repo_uuid = $repository_api->getRepositorySVNUUID();
+    } else if ($repository_api instanceof ArcanistMercurialAPI) {
+      // TODO: Provide this information.
+    } else {
+      throw new Exception("Unsupported repository API!");
     }
 
     $working_copy = $this->getWorkingCopy();
@@ -527,10 +531,18 @@ EOTEXT
   }
 
   protected function shouldOnlyCreateDiff() {
+
     $repository_api = $this->getRepositoryAPI();
     if ($repository_api instanceof ArcanistSubversionAPI) {
       return true;
     }
+
+    if ($repository_api instanceof ArcanistMercurialAPI) {
+      // TODO: This is unlikely to be correct since it excludes using local
+      // branching in Mercurial.
+      return true;
+    }
+
     return $this->getArgument('preview') ||
            $this->getArgument('only');
   }
@@ -580,11 +592,19 @@ EOTEXT
         }
       }
 
-    } else {
+    } else if ($repository_api instanceof ArcanistGitAPI) {
       $this->parseGitRelativeCommit(
         $repository_api,
         $this->getArgument('paths', array()));
       $paths = $repository_api->getWorkingCopyStatus();
+    } else if ($repository_api instanceof ArcanistMercurialAPI) {
+      // TODO: Unify this and the previous block.
+
+      // TODO: Parse the relative commit.
+
+      $paths = $repository_api->getWorkingCopyStatus();
+    } else {
+      throw new Exception("Unknown VCS!");
     }
 
     foreach ($paths as $path => $mask) {
@@ -669,6 +689,9 @@ EOTEXT
       }
       $changes = $parser->parseDiff($diff);
 
+    } else if ($repository_api instanceof ArcanistMercurialAPI) {
+      $diff = $repository_api->getFullMercurialDiff();
+      $changes = $parser->parseDiff($diff);
     } else {
       throw new Exception("Repository API is not supported.");
     }