diff --git a/src/lint/linter/xhpast/ArcanistXHPASTLinter.php b/src/lint/linter/xhpast/ArcanistXHPASTLinter.php
index f78cd3ab..9e129541 100644
--- a/src/lint/linter/xhpast/ArcanistXHPASTLinter.php
+++ b/src/lint/linter/xhpast/ArcanistXHPASTLinter.php
@@ -17,7 +17,7 @@
  */
 
 /**
- * Uses XHPAST to apply lint rules to PHP or PHP+XHP.
+ * Uses XHPAST to apply lint rules to PHP.
  *
  * @group linter
  */
diff --git a/src/unit/engine/base/ArcanistBaseUnitTestEngine.php b/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
index 0668f61d..dc7f492b 100644
--- a/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
+++ b/src/unit/engine/base/ArcanistBaseUnitTestEngine.php
@@ -1,7 +1,7 @@
 <?php
 
 /*
- * Copyright 2011 Facebook, Inc.
+ * Copyright 2012 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,7 @@ abstract class ArcanistBaseUnitTestEngine {
   private $arguments = array();
   protected $diffID;
   private $enableAsyncTests;
+  private $enableCoverage;
 
   final public function __construct() {
 
@@ -70,6 +71,15 @@ abstract class ArcanistBaseUnitTestEngine {
     return $this->enableAsyncTests;
   }
 
+  final public function setEnableCoverage($enable_coverage) {
+    $this->enableCoverage = $enable_coverage;
+    return $this;
+  }
+
+  final public function getEnableCoverage() {
+    return $this->enableCoverage;
+  }
+
   abstract public function run();
 
   /**
diff --git a/src/unit/engine/phutil/PhutilUnitTestEngine.php b/src/unit/engine/phutil/PhutilUnitTestEngine.php
index 7f8f5199..7cef6fae 100644
--- a/src/unit/engine/phutil/PhutilUnitTestEngine.php
+++ b/src/unit/engine/phutil/PhutilUnitTestEngine.php
@@ -118,17 +118,34 @@ final class PhutilUnitTestEngine extends ArcanistBaseUnitTestEngine {
         "No tests to run. You may need to rebuild the phutil library map.");
     }
 
+    $enable_coverage = $this->getEnableCoverage();
+    if ($enable_coverage !== false) {
+      if (!function_exists('xdebug_start_code_coverage')) {
+        if ($enable_coverage === true) {
+          throw new ArcanistUsageException(
+            "You specified --coverage but xdebug is not available, so ".
+            "coverage can not be enabled for PhutilUnitTestEngine.");
+        }
+      } else {
+        $enable_coverage = true;
+      }
+    }
+
     $results = array();
     foreach ($run_tests as $test_class) {
       PhutilSymbolLoader::loadClass($test_class);
       $test_case = newv($test_class, array());
+      $test_case->setEnableCoverage($enable_coverage);
+      $test_case->setProjectRoot($this->getWorkingCopy()->getProjectRoot());
+      $test_case->setPaths($this->getPaths());
       $results[] = $test_case->run();
     }
+
+
     if ($results) {
       $results = call_user_func_array('array_merge', $results);
     }
 
-
     return $results;
   }
 
diff --git a/src/unit/engine/phutil/__init__.php b/src/unit/engine/phutil/__init__.php
index c7db7cc4..58bd4ab3 100644
--- a/src/unit/engine/phutil/__init__.php
+++ b/src/unit/engine/phutil/__init__.php
@@ -6,6 +6,7 @@
 
 
 
+phutil_require_module('arcanist', 'exception/usage');
 phutil_require_module('arcanist', 'exception/usage/noeffect');
 phutil_require_module('arcanist', 'unit/engine/base');
 
diff --git a/src/unit/engine/phutil/testcase/ArcanistPhutilTestCase.php b/src/unit/engine/phutil/testcase/ArcanistPhutilTestCase.php
index a7e6e1f7..e306c03f 100644
--- a/src/unit/engine/phutil/testcase/ArcanistPhutilTestCase.php
+++ b/src/unit/engine/phutil/testcase/ArcanistPhutilTestCase.php
@@ -30,6 +30,10 @@ abstract class ArcanistPhutilTestCase {
   private $runningTest;
   private $testStartTime;
   private $results = array();
+  private $enableCoverage;
+  private $coverage = array();
+  private $projectRoot;
+  private $paths;
 
 
 /* -(  Making Test Assertions  )--------------------------------------------- */
@@ -179,7 +183,10 @@ abstract class ArcanistPhutilTestCase {
    * @task internal
    */
   final private function failTest($reason) {
+    $coverage = $this->endCoverage();
+
     $result = new ArcanistUnitTestResult();
+    $result->setCoverage($coverage);
     $result->setName($this->runningTest);
     $result->setResult(ArcanistUnitTestResult::RESULT_FAIL);
     $result->setDuration(microtime(true) - $this->testStartTime);
@@ -197,7 +204,10 @@ abstract class ArcanistPhutilTestCase {
    * @task internal
    */
   final private function passTest($reason) {
+    $coverage = $this->endCoverage();
+
     $result = new ArcanistUnitTestResult();
+    $result->setCoverage($coverage);
     $result->setName($this->runningTest);
     $result->setResult(ArcanistUnitTestResult::RESULT_PASS);
     $result->setDuration(microtime(true) - $this->testStartTime);
@@ -233,6 +243,7 @@ abstract class ArcanistPhutilTestCase {
         try {
           $this->willRunOneTest($name);
 
+          $this->beginCoverage();
           $test_exception = null;
           try {
             call_user_func_array(
@@ -263,4 +274,76 @@ abstract class ArcanistPhutilTestCase {
     return $this->results;
   }
 
+  final public function setEnableCoverage($enable_coverage) {
+    $this->enableCoverage = $enable_coverage;
+    return $this;
+  }
+
+  final private function beginCoverage() {
+    if (!$this->enableCoverage) {
+      return;
+    }
+
+    $this->assertCoverageAvailable();
+    xdebug_start_code_coverage(XDEBUG_CC_UNUSED | XDEBUG_CC_DEAD_CODE);
+  }
+
+  final private function endCoverage() {
+    if (!$this->enableCoverage) {
+      return;
+    }
+
+    $result = xdebug_get_code_coverage();
+    xdebug_stop_code_coverage($cleanup = false);
+
+    $coverage = array();
+
+    foreach ($result as $file => $report) {
+      if (strncmp($file, $this->projectRoot, strlen($this->projectRoot))) {
+        continue;
+      }
+
+      $max = max(array_keys($report));
+      $str = '';
+      for ($ii = 1; $ii <= $max; $ii++) {
+        $c = idx($report, $ii);
+        if ($c === -1) {
+          $str .= 'U'; // Un-covered.
+        } else if ($c === -2) {
+          // TODO: This indicates "unreachable", but it flags the closing braces
+          // of functions which end in "return", which is super ridiculous. Just
+          // ignore it for now.
+          $str .= 'N'; // Not executable.
+        } else if ($c === 1) {
+          $str .= 'C'; // Covered.
+        } else {
+          $str .= 'N'; // Not executable.
+        }
+      }
+      $coverage[substr($file, strlen($this->projectRoot) + 1)] = $str;
+    }
+
+    // Only keep coverage information for files modified by the change.
+    $coverage = array_select_keys($coverage, $this->paths);
+
+    return $coverage;
+  }
+
+  final private function assertCoverageAvailable() {
+    if (!function_exists('xdebug_start_code_coverage')) {
+      throw new Exception(
+        "You've enabled code coverage but XDebug is not installed.");
+    }
+  }
+
+  final public function setProjectRoot($project_root) {
+    $this->projectRoot = $project_root;
+    return $this;
+  }
+
+  final public function setPaths(array $paths) {
+    $this->paths = $paths;
+    return $this;
+  }
+
 }
diff --git a/src/unit/result/ArcanistUnitTestResult.php b/src/unit/result/ArcanistUnitTestResult.php
index 688dc894..2ba78665 100644
--- a/src/unit/result/ArcanistUnitTestResult.php
+++ b/src/unit/result/ArcanistUnitTestResult.php
@@ -35,6 +35,7 @@ final class ArcanistUnitTestResult {
   private $result;
   private $duration;
   private $userData;
+  private $coverage;
 
   public function setName($name) {
     $this->name = $name;
@@ -72,4 +73,32 @@ final class ArcanistUnitTestResult {
     return $this->userData;
   }
 
+  public function setCoverage($coverage) {
+    $this->coverage = $coverage;
+    return $this;
+  }
+
+  public function getCoverage() {
+    return $this->coverage;
+  }
+
+  /**
+   * Merge several coverage reports into a comprehensive coverage report.
+   *
+   * @param list List of coverage report strings.
+   * @return string Cumulative coverage report.
+   */
+  public static function mergeCoverage(array $coverage) {
+    $base = reset($coverage);
+    foreach ($coverage as $more_coverage) {
+      $len = min(strlen($base), strlen($more_coverage));
+      for ($ii = 0; $ii < $len; $ii++) {
+        if ($more_coverage[$ii] == 'C') {
+          $base[$ii] = 'C';
+        }
+      }
+    }
+    return $base;
+  }
+
 }
diff --git a/src/workflow/diff/ArcanistDiffWorkflow.php b/src/workflow/diff/ArcanistDiffWorkflow.php
index 49d28bcf..74943705 100644
--- a/src/workflow/diff/ArcanistDiffWorkflow.php
+++ b/src/workflow/diff/ArcanistDiffWorkflow.php
@@ -30,7 +30,7 @@ final class ArcanistDiffWorkflow extends ArcanistBaseWorkflow {
 
   private $hasWarnedExternals = false;
   private $unresolvedLint;
-  private $unresolvedTests;
+  private $testResults;
   private $diffID;
   private $unitWorkflow;
 
@@ -1067,7 +1067,7 @@ EOTEXT
           break;
       }
 
-      $this->unresolvedTests = $this->unitWorkflow->getUnresolvedTests();
+      $this->testResults = $this->unitWorkflow->getTestResults();
 
       return $unit_result;
     } catch (ArcanistNoEngineException $ex) {
@@ -1571,16 +1571,17 @@ EOTEXT
    * @task diffprop
    */
   private function updateUnitDiffProperty() {
-    if (!$this->unresolvedTests) {
+    if (!$this->testResults) {
       return;
     }
 
     $data = array();
-    foreach ($this->unresolvedTests as $test) {
+    foreach ($this->testResults as $test) {
       $data[] = array(
         'name'      => $test->getName(),
         'result'    => $test->getResult(),
         'userdata'  => $test->getUserData(),
+        'coverage'  => $test->getCoverage(),
       );
     }
 
diff --git a/src/workflow/unit/ArcanistUnitWorkflow.php b/src/workflow/unit/ArcanistUnitWorkflow.php
index 4c4c1259..3a2cda6a 100644
--- a/src/workflow/unit/ArcanistUnitWorkflow.php
+++ b/src/workflow/unit/ArcanistUnitWorkflow.php
@@ -30,6 +30,7 @@ final class ArcanistUnitWorkflow extends ArcanistBaseWorkflow {
   const RESULT_POSTPONED = 4;
 
   private $unresolvedTests;
+  private $testResults;
   private $engine;
 
   public function getCommandHelp() {
@@ -61,6 +62,19 @@ EOTEXT
         'help' =>
           "Override configured unit engine for this project."
       ),
+      'coverage' => array(
+        'help' => 'Always enable coverage information.',
+        'conflicts' => array(
+          'no-coverage' => null,
+        ),
+      ),
+      'no-coverage' => array(
+        'help' => 'Always disable coverage information.',
+      ),
+      'detailed-coverage' => array(
+        'help' => "Show a detailed coverage report on the CLI. Implies ".
+                  "--coverage.",
+      ),
       '*' => 'paths',
     );
   }
@@ -108,6 +122,15 @@ EOTEXT
     $this->engine->setPaths($paths);
     $this->engine->setArguments($this->getPassthruArgumentsAsMap('unit'));
 
+    $enable_coverage = null; // Means "default".
+    if ($this->getArgument('coverage') ||
+        $this->getArgument('detailed-coverage')) {
+      $enable_coverage = true;
+    } else if ($this->getArgument('no-coverage')) {
+      $enable_coverage = false;
+    }
+    $this->engine->setEnableCoverage($enable_coverage);
+
     // Enable possible async tests only for 'arc diff' not 'arc unit'
     if ($this->getParentWorkflow()) {
       $this->engine->setEnableAsyncTests(true);
@@ -116,6 +139,7 @@ EOTEXT
     }
 
     $results = $this->engine->run();
+    $this->testResults = $results;
 
     $status_codes = array(
       ArcanistUnitTestResult::RESULT_PASS => phutil_console_format(
@@ -133,6 +157,7 @@ EOTEXT
       );
 
     $unresolved = array();
+    $coverage = array();
     $postponed_count = 0;
     foreach ($results as $result) {
       $result_code = $result->getResult();
@@ -154,6 +179,11 @@ EOTEXT
           $unresolved[] = $result;
         }
       }
+      if ($result->getCoverage()) {
+        foreach ($result->getCoverage() as $file => $report) {
+          $coverage[$file][] = $report;
+        }
+      }
     }
     if ($postponed_count) {
       echo sprintf("%s %d %s\n",
@@ -162,6 +192,40 @@ EOTEXT
          ($postponed_count > 1)?'tests':'test');
     }
 
+    if ($coverage) {
+      $file_coverage = array();
+      $file_reports = array();
+      foreach ($coverage as $file => $reports) {
+        $report = ArcanistUnitTestResult::mergeCoverage($reports);
+        $cov = substr_count($report, 'C');
+        $uncov = substr_count($report, 'U');
+        if ($cov + $uncov) {
+          $coverage = $cov / ($cov + $uncov);
+        } else {
+          $coverage = 0;
+        }
+        $file_coverage[$file] = $coverage;
+        $file_reports[$file] = $report;
+      }
+      echo "\n";
+      echo phutil_console_format('__COVERAGE REPORT__');
+      echo "\n";
+
+      asort($file_coverage);
+      foreach ($file_coverage as $file => $coverage) {
+        echo phutil_console_format(
+          "    **%s%%**     %s\n",
+          sprintf('% 3d', (int)(100 * $coverage)),
+          $file);
+
+        if ($this->getArgument('detailed-coverage')) {
+          echo $this->renderDetailedCoverageReport(
+            $file,
+            $file_reports[$file]);
+        }
+      }
+    }
+
     $this->unresolvedTests = $unresolved;
 
     $overall_result = self::RESULT_OKAY;
@@ -186,6 +250,10 @@ EOTEXT
     return $this->unresolvedTests;
   }
 
+  public function getTestResults() {
+    return $this->testResults;
+  }
+
   public function setDifferentialDiffID($id) {
     if ($this->engine) {
       $this->engine->setDifferentialDiffID($id);
@@ -229,4 +297,46 @@ EOTEXT
 
     return ' <1ms';
   }
+
+  private function renderDetailedCoverageReport($file, $report) {
+    $data = $this->getRepositoryAPI()->getCurrentFileData($file);
+    $data = explode("\n", $data);
+
+    $out = '';
+
+    $n = 0;
+    foreach ($data as $line) {
+      $out .= sprintf('% 5d  ', $n + 1);
+      $line = str_pad($line, 80, ' ');
+      if (empty($report[$n])) {
+        $c = 'N';
+      } else {
+        $c = $report[$n];
+      }
+      switch ($c) {
+        case 'C':
+          $out .= phutil_console_format(
+            '<bg:green> %s </bg>',
+            $line);
+          break;
+        case 'U':
+          $out .= phutil_console_format(
+            '<bg:red> %s </bg>',
+            $line);
+          break;
+        case 'X':
+          $out .= phutil_console_format(
+            '<bg:magenta> %s </bg>',
+            $line);
+          break;
+        default:
+          $out .= ' '.$line.' ';
+          break;
+      }
+      $out .= "\n";
+      $n++;
+    }
+
+    return $out;
+  }
 }