Why can't I see my UI in the scene view?

Symptoms

•  None of the UI elements are showing in the scene view.

Cause

• There is a bug prior to Unity 5.3, in which the UI is now visible in the Scene View. 
• The UI layer is hidden. 

Resolution

• Update to Unity 5.3 or greater. 
• If the UI Layer is hidden, un-hide it. To do so just click on the Layers dropdown to the upper right of Unity's Editor and click on the layer you wish to un-hide.

More Information

• http://issuetracker.unity3d.com/issues/sceneview-ui-no-longer-renders-in-the-scene-view
• http://answers.unity3d.com/questions/830367/why-cant-see-ui-elements-in-scene-view.html
• http://answers.unity3d.com/questions/964113/why-ui-elements-are-invisible-on-scene-view.html
• http://docs.unity3d.com/Manual/Layers.html

This article applies to Unity versions 5.x 5+