I just happened to know that Start() can return IEnumerator to be treated as a coroutine, but I can no longer find this in the documentation. I was curious if any other Unity messages could return IEnumerator, and it took me some digging through the forums to find this: Most Unity callbacks can be coroutines. Unity automatically starts it as coroutine if it returns an IEnumerator. Only a few can't be a coroutine. Most commonly: Awake, Update, LateUpdate, FixedUpdate, OnGUI, OnEnable, OnDisable, OnDestroy. I probably forgot some others, however it's usually stated for each callback if it can be a coroutine or not. (source: https://answers.unity.com/questions/754968/can-monobehaviorstart-return-an-ienumerator-hint-y.html) I think this would be great to mention to the Monobehaviour Scripting API page, Coroutine Manual page, and possibly the Monobehaviour.Start() documentation.