diff --git a/.gitignore b/.gitignore
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 # Diviner
 /docs/
 /.divinercache/
+/src/.cache/
 
 # libphutil
 /src/.phutil_module_cache
diff --git a/src/infrastructure/internationalization/management/PhabricatorInternationalizationManagementExtractWorkflow.php b/src/infrastructure/internationalization/management/PhabricatorInternationalizationManagementExtractWorkflow.php
--- a/src/infrastructure/internationalization/management/PhabricatorInternationalizationManagementExtractWorkflow.php
+++ b/src/infrastructure/internationalization/management/PhabricatorInternationalizationManagementExtractWorkflow.php
@@ -3,9 +3,13 @@
 final class PhabricatorInternationalizationManagementExtractWorkflow
   extends PhabricatorInternationalizationManagementWorkflow {
 
+  const CACHE_VERSION = 1;
+
   protected function didConstruct() {
     $this
       ->setName('extract')
+      ->setExamples(
+        '**extract** [__options__] __library__')
       ->setSynopsis(pht('Extract translatable strings.'))
       ->setArguments(
         array(
@@ -13,44 +17,138 @@
             'name' => 'paths',
             'wildcard' => true,
           ),
+          array(
+            'name' => 'clean',
+            'help' => pht('Drop caches before extracting strings. Slow!'),
+          ),
         ));
   }
 
   public function execute(PhutilArgumentParser $args) {
     $console = PhutilConsole::getConsole();
+
     $paths = $args->getArg('paths');
+    if (!$paths) {
+      $paths = array(getcwd());
+    }
 
-    $futures = array();
+    $targets = array();
     foreach ($paths as $path) {
       $root = Filesystem::resolvePath($path);
-      $path_files = id(new FileFinder($root))
-        ->withType('f')
-        ->withSuffix('php')
+
+      if (!Filesystem::pathExists($root) || !is_dir($root)) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Path "%s" does not exist, or is not a directory.',
+            $path));
+      }
+
+      $libraries = id(new FileFinder($path))
+        ->withPath('*/__phutil_library_init__.php')
         ->find();
+      if (!$libraries) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Path "%s" contains no libphutil libraries.',
+            $path));
+      }
 
-      foreach ($path_files as $file) {
-        $full_path = $root.DIRECTORY_SEPARATOR.$file;
-        $data = Filesystem::readFile($full_path);
-        $futures[$full_path] = PhutilXHPASTBinary::getParserFuture($data);
+      foreach ($libraries as $library) {
+        $targets[] = Filesystem::resolvePath(dirname($library)).'/';
       }
     }
 
-    $console->writeErr(
-      "%s\n",
-      pht('Found %s file(s)...', phutil_count($futures)));
+    $targets = array_unique($targets);
 
-    $results = array();
+    foreach ($targets as $library) {
+      echo tsprintf(
+        "**<bg:blue> %s </bg>** %s\n",
+        pht('EXTRACT'),
+        pht(
+          'Extracting "%s"...',
+          Filesystem::readablePath($library)));
+
+      $this->extractLibrary($library);
+    }
+
+    return 0;
+  }
+
+  private function extractLibrary($root) {
+    $files = $this->loadLibraryFiles($root);
+    $cache = $this->readCache($root);
+
+    $modified = $this->getModifiedFiles($files, $cache);
+    $cache['files'] = $files;
+
+    if ($modified) {
+      echo tsprintf(
+        "**<bg:blue> %s </bg>** %s\n",
+        pht('MODIFIED'),
+        pht(
+          'Found %s modified file(s) (of %s total).',
+          phutil_count($modified),
+          phutil_count($files)));
+
+      $old_strings = idx($cache, 'strings');
+      $old_strings = array_select_keys($old_strings, $files);
+      $new_strings = $this->extractFiles($root, $modified);
+      $all_strings = $new_strings + $old_strings;
+      $cache['strings'] = $all_strings;
+
+      $this->writeStrings($root, $all_strings);
+    } else {
+      echo tsprintf(
+        "**<bg:blue> %s </bg>** %s\n",
+        pht('NOT MODIFIED'),
+        pht('Strings for this library are already up to date.'));
+    }
+
+    $cache = id(new PhutilJSON())->encodeFormatted($cache);
+    $this->writeCache($root, 'i18n_files.json', $cache);
+  }
+
+  private function getModifiedFiles(array $files, array $cache) {
+    $known = idx($cache, 'files', array());
+    $known = array_fuse($known);
+
+    $modified = array();
+    foreach ($files as $file => $hash) {
+
+      if (isset($known[$hash])) {
+        continue;
+      }
+      $modified[$file] = $hash;
+    }
+
+    return $modified;
+  }
+
+  private function extractFiles($root_path, array $files) {
+    $hashes = array();
+
+    $futures = array();
+    foreach ($files as $file => $hash) {
+      $full_path = $root_path.DIRECTORY_SEPARATOR.$file;
+      $data = Filesystem::readFile($full_path);
+      $futures[$full_path] = PhutilXHPASTBinary::getParserFuture($data);
+
+      $hashes[$full_path] = $hash;
+    }
 
     $bar = id(new PhutilConsoleProgressBar())
       ->setTotal(count($futures));
 
     $messages = array();
+    $results = array();
 
     $futures = id(new FutureIterator($futures))
       ->limit(8);
     foreach ($futures as $full_path => $future) {
       $bar->update(1);
 
+      $hash = $hashes[$full_path];
+
       try {
         $tree = XHPASTTree::newFromDataAndResolvedExecFuture(
           Filesystem::readFile($full_path),
@@ -67,24 +165,27 @@
       $calls = $root->selectDescendantsOfType('n_FUNCTION_CALL');
       foreach ($calls as $call) {
         $name = $call->getChildByIndex(0)->getConcreteString();
-        if ($name == 'pht') {
-          $params = $call->getChildByIndex(1, 'n_CALL_PARAMETER_LIST');
-          $string_node = $params->getChildByIndex(0);
-          $string_line = $string_node->getLineNumber();
-          try {
-            $string_value = $string_node->evalStatic();
-
-            $results[$string_value][] = array(
-              'file' => Filesystem::readablePath($full_path),
-              'line' => $string_line,
-            );
-          } catch (Exception $ex) {
-            $messages[] = pht(
-              'WARNING: Failed to evaluate pht() call on line %d in "%s": %s',
-              $call->getLineNumber(),
-              $full_path,
-              $ex->getMessage());
-          }
+        if ($name != 'pht') {
+          continue;
+        }
+
+        $params = $call->getChildByIndex(1, 'n_CALL_PARAMETER_LIST');
+        $string_node = $params->getChildByIndex(0);
+        $string_line = $string_node->getLineNumber();
+        try {
+          $string_value = $string_node->evalStatic();
+
+          $results[$hash][] = array(
+            'string' => $string_value,
+            'file' => Filesystem::readablePath($full_path, $root_path),
+            'line' => $string_line,
+          );
+        } catch (Exception $ex) {
+          $messages[] = pht(
+            'WARNING: Failed to evaluate pht() call on line %d in "%s": %s',
+            $call->getLineNumber(),
+            $full_path,
+            $ex->getMessage());
         }
       }
 
@@ -93,28 +194,109 @@
     $bar->done();
 
     foreach ($messages as $message) {
-      $console->writeErr("%s\n", $message);
+      echo tsprintf(
+        "**<bg:yellow> %s </bg>** %s\n",
+        pht('WARNING'),
+        $message);
     }
 
-    ksort($results);
+    return $results;
+  }
 
-    $out = array();
-    $out[] = '<?php';
-    $out[] = '// @no'.'lint';
-    $out[] = 'return array(';
-    foreach ($results as $string => $locations) {
-      foreach ($locations as $location) {
-        $out[] = '  // '.$location['file'].':'.$location['line'];
+  private function writeStrings($root, array $strings) {
+    $map = array();
+    foreach ($strings as $hash => $string_list) {
+      foreach ($string_list as $string_info) {
+        $map[$string_info['string']]['uses'][] = array(
+          'file' => $string_info['file'],
+          'line' => $string_info['line'],
+        );
       }
-      $out[] = "  '".addcslashes($string, "\0..\37\\'\177..\377")."' => null,";
-      $out[] = null;
     }
-    $out[] = ');';
-    $out[] = null;
 
-    echo implode("\n", $out);
+    ksort($map);
 
-    return 0;
+    $json = id(new PhutilJSON())->encodeFormatted($map);
+    $this->writeCache($root, 'i18n_strings.json', $json);
+  }
+
+  private function loadLibraryFiles($root) {
+    $files = id(new FileFinder($root))
+      ->withType('f')
+      ->withSuffix('php')
+      ->excludePath('*/.*')
+      ->setGenerateChecksums(true)
+      ->find();
+
+    $map = array();
+    foreach ($files as $file => $hash) {
+      $file = Filesystem::readablePath($file, $root);
+      $file = ltrim($file, '/');
+
+      if (dirname($file) == '.') {
+        continue;
+      }
+
+      if (dirname($file) == 'extensions') {
+        continue;
+      }
+
+      $map[$file] = md5($hash.$file);
+    }
+
+    return $map;
+  }
+
+  private function readCache($root) {
+    $path = $this->getCachePath($root, 'i18n_files.json');
+
+    $default = array(
+      'version' => self::CACHE_VERSION,
+      'files' => array(),
+      'strings' => array(),
+    );
+
+    if ($this->getArgv()->getArg('clean')) {
+      return $default;
+    }
+
+    if (!Filesystem::pathExists($path)) {
+      return $default;
+    }
+
+    try {
+      $data = Filesystem::readFile($path);
+    } catch (Exception $ex) {
+      return $default;
+    }
+
+    try {
+      $cache = phutil_json_decode($data);
+    } catch (PhutilJSONParserException $e) {
+      return $default;
+    }
+
+    $version = idx($cache, 'version');
+    if ($version !== self::CACHE_VERSION) {
+      return $default;
+    }
+
+    return $cache;
+  }
+
+  private function writeCache($root, $file, $data) {
+    $path = $this->getCachePath($root, $file);
+
+    $cache_dir = dirname($path);
+    if (!Filesystem::pathExists($cache_dir)) {
+      Filesystem::createDirectory($cache_dir, 0755, true);
+    }
+
+    Filesystem::writeFile($path, $data);
+  }
+
+  private function getCachePath($root, $to_file) {
+    return $root.'/.cache/'.$to_file;
   }
 
 }