diff --git a/src/filesystem/Filesystem.php b/src/filesystem/Filesystem.php
--- a/src/filesystem/Filesystem.php
+++ b/src/filesystem/Filesystem.php
@@ -853,6 +853,53 @@
   }
 
   /**
+   * Retrieve the relative location between two absolute paths.
+   *
+   * This function returns the relative path between two given absolute paths.
+   * The implementation was based on a post on
+   * [[http://stackoverflow.com/a/2638272/1369417 | StackOverflow]].
+   *
+   * @param  string  The source destination.
+   * @param  string  The target destination.
+   * @return string  The relative path between the source and target
+   *                 destinations.
+   * @task   path
+   */
+  public static function relativePath($from, $to) {
+    // Some compatibility fixes for Windows paths.
+    $from = is_dir($from) ? rtrim($from, '\/').'/' : $from;
+    $to   = is_dir($to)   ? rtrim($to,   '\/').'/' : $to;
+    $from = str_replace('\\', '/', $from);
+    $to   = str_replace('\\', '/', $to);
+
+    $from = explode('/', $from);
+    $to   = explode('/', $to);
+
+    $rel_path = $to;
+
+    foreach ($from as $depth => $dir) {
+      // Find first non-matching directory.
+      if ($dir === $to[$depth]) {
+        // Ignore this directory.
+        array_shift($rel_path);
+      } else {
+        // Get number of remaining directories to $from.
+        $remaining = count($from) - $depth;
+        if ($remaining > 1) {
+          // Add traversals up to first matching directory.
+          $pad_length = (count($rel_path) + $remaining - 1) * -1;
+          $rel_path = array_pad($rel_path, $pad_length, '..');
+          break;
+        }
+      }
+    }
+
+    // `DIRECTORY_SEPARATOR` is not necessary. See
+    // http://us2.php.net/manual/en/ref.filesystem.php#73954.
+    return implode('/', $rel_path);
+  }
+
+  /**
    * Test whether a path is descendant from some root path after resolving all
    * symlinks and removing artifacts. Both paths must exists for the relation
    * to obtain. A path is always a descendant of itself as long as it exists.
diff --git a/src/filesystem/__tests__/FilesystemTestCase.php b/src/filesystem/__tests__/FilesystemTestCase.php
--- a/src/filesystem/__tests__/FilesystemTestCase.php
+++ b/src/filesystem/__tests__/FilesystemTestCase.php
@@ -121,45 +121,47 @@
     }
   }
 
-  public function testisDescendant() {
+  public function testRelativePath() {
     $test_cases = array(
       array(
-        __FILE__,
-        dirname(__FILE__),
-        true,
+        '/',
+        '/foo/bar/baz',
+        'foo/bar/baz',
       ),
       array(
-        dirname(__FILE__),
-        dirname(dirname(__FILE__)),
-        true,
+        '/foo',
+        '/foo/bar/baz',
+        'bar/baz',
       ),
       array(
-        dirname(__FILE__),
-        phutil_get_library_root_for_path(__FILE__),
-        true,
+        '/foo/bar/baz',
+        '/foo/foobar',
+        '../foobar',
       ),
+
+      // Windows paths
       array(
-        dirname(dirname(__FILE__)),
-        dirname(__FILE__),
-        false,
+        'c:\\',
+        'c:\\Windows\\System32\\Drivers\\etc\\hosts',
+        'Windows/System32/Drivers/etc/hosts',
       ),
       array(
-        dirname(__FILE__).'/quack',
-        dirname(__FILE__),
-        false,
+        'c:\\Users\\Bill Gates\\',
+        'c:\\Windows\\System32\\Drivers\\etc\\hosts',
+        '../../Windows/System32/Drivers/etc/hosts',
       ),
     );
 
     foreach ($test_cases as $test_case) {
-      list($path, $root, $expected) = $test_case;
+      list($from, $to, $expected) = $test_case;
 
       $this->assertEqual(
         $expected,
-        Filesystem::isDescendant($path, $root),
+        Filesystem::relativePath($from, $to),
         sprintf(
-          'Filesystem::isDescendant(%s, %s)',
-          phutil_var_export($path),
-          phutil_var_export($root)));
+          'Filesystem::relativePath(%s, %s)',
+          phutil_var_export($from),
+          phutil_var_export($to)));
     }
   }