diff --git a/lib/zuul.js b/lib/zuul.js
index <HASH>..<HASH> 100644
--- a/lib/zuul.js
+++ b/lib/zuul.js
@@ -28,6 +28,8 @@ function Zuul(config) {
 
     // list of browsers to test
     self._browsers = [];
+
+    self._concurrency = config.concurrency || 3;
 };
 
 Zuul.prototype.__proto__ = EventEmitter.prototype;
@@ -92,8 +94,7 @@ Zuul.prototype.run = function(done) {
         }
 
         var batch = new Batch();
-        // TODO(shtylman) option
-        batch.concurrency(3);
+        batch.concurrency(self._concurrency);
 
         var passed = true;
add concurrency option to specify how many browsers to run at once