diff --git a/lib/index.js b/lib/index.js
index 534544f4bd0..ac242add2a9 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -65,6 +65,8 @@ var util = require('./common/util.js');
  * @constructor
  *
  * @param {object} config - Connection configuration options.
+ * @param {string} config.projectId - The project ID from the Google Developer's
+ *     Console. E.g. 'purple-grape-123'.
  * @param {string=} config.keyFilename - Full path to the a .json, .pem, or .p12
  *     key downloaded from the Google Developers Console. NOTE: .pem and .p12
  *     require you to specify `config.email` as well.