Updated Event Execution Order

Author: Michael Garforth

''Please note that this article supersedes an earlier version that was written by Joachim Ante of Unity Technologies in a forum post. This new article is more comprehensive and is more accurate overall. However, as the information has been drawn from observation rather than inside knowledge, it may be that there are aspects that are incorrect. If you discover inaccuracies, please describe your findings in the talk page.''

Game loop
The following all happens in a single loop, in the order specified Physics does not necessarily run each time, and can run multiple times

After an instantiation
For objects that were instantiated last game loop iteration:
 * Start

Physics loop
Physics is run in a loop, iterating over the following until physics has caught up to the current frame:
 * FixedUpdate
 * Physics simulation
 * OnCollisionEnter/OnCollisionStay/OnCollisionExit
 * OnTriggerEnter/OnTriggerStay/OnTriggerExit
 * WaitForFixedUpdate (Coroutines which yielded WaitForFixedUpdate will resume execution after the next physics iteration completes)

Update events
The rest of the events are run once per game loop iteration:
 * OnMouseDown/OnMouseUp/OnMouseDrag/OnMouseEnter/OnMouseExit/OnMouseOver
 * Update
 * WaitForSeconds/yield/yield return 0 (Coroutines which yielded one of these will resume execution here, with WaitForSeconds resuming after the defined time)
 * LateUpdate

Rendering

 * OnPreCull
 * OnBecameVisible/OnBecameInvisible
 * OnWillRenderObject
 * OnPreRender
 * OnRenderObject
 * OnPostRender
 * OnRenderImage (Pro only)


 * OnGUI (Called multiple times. Layout then Repaint events are called first, followed by a Layout and keyboard/mouse event for each input event)
 * OnDrawGizmos (Only when scene is drawn, even if object is inactive)

Before the frame ends

 * WaitForEndOfFrame (Coroutines which yielded WaitForEndOfFrame will resume execution before the next game loop finishes)

First scene load
Repeated once per object:
 * Awake
 * OnEnable (If the object is active)

At the beginning of the next frame, per object:
 * Start

Before the level loads
Repeated once per active object, after the end of the current frame:
 * OnDisable

After level loads
Repeated once per scene object:
 * Awake
 * OnEnable (If the object is active)

Repeated once per active DontDestroyOnLoad object from last level:
 * OnEnable

Repeated once per active object:
 * OnLevelWasLoaded

When quitting
For all active objects:
 * OnApplicationQuit
 * OnDisable

Immediate events
These are all called immediately when certain functions are called (e.g. Instantiate, setting enabled):
 * Awake - called during MonoBehaviour instantiation
 * OnEnable/OnDisable - OnEnable called during MonoBehaviour instantiation
 * Reset - called when the script is reset (Editor Only)