4 or 5 Making a button transparent

As already mentioned - DO NOT use transparent images or similar tricks. A fully transparent image still renders at exactly the same cost as a regular one, so it's a very bad idea.

Improving on the answer by SignalZak, there are 2 options. For both of them, use the EventTrigger component to handle events.

• the lazy one, no coding required: Create a Text object with no text and set the rectangle to the desired size.

• the proper one (just a few lines of code): Create a custom UI control, derived from UnityEngine.UI.Graphic, and override Raycast to always return true (testing against the rect bounds was done already), and also OnFillVBO to do nothing (the default implementation creates a white opaque quad to fill the rectangle).

     public class Touchable : Graphic
       {
           public override bool Raycast(Vector2 sp, Camera eventCamera)
           {
               //return base.Raycast(sp, eventCamera);
               return true;
           }
   
           protected override void OnFillVBO(List<UIVertex> vbo)
           {
               //base.OnFillVBO(vbo);
           }
       }
  
  
  
   [CustomEditor(typeof(Touchable))]
       public class TouchableEditor : Editor
       {
           public override void OnInspectorGUI()
           {
               // nothing
           }
       }
  
  

The answers above are close, but not working because the button DOES NOT WORK once you delete or take out the invisible button.

Steps to having a WORKING invisible button:

1. Create the button as a UI Button

2. Delete the Text property that the button has assigned to it

3. Go to the Button's "Image (Script)", change the sprite to UIMask. UIMask is used as a placeholder to hide elements but they will still be there

I hope this helps sum things up.

Updated @bcristian 's Touchable for Unity 5.3+

 // Touchable.cs
 using UnityEngine;
 
 namespace UnityEngine.UI {
 
     public class Touchable : Graphic {
         
         public override bool Raycast(Vector2 sp, Camera eventCamera) {
             return true;
         }
 
         protected override void OnPopulateMesh(VertexHelper vh) {
             vh.Clear();
         }
     }
 }
 
 
 // TouchableEditor.cs
 using UnityEngine;
 using UnityEditor;
 
 namespace UnityEngine.UI {
 
     [CustomEditor(typeof(Touchable))]
     public class TouchableEditor : Editor {
 
         public override void OnInspectorGUI() {}
     }
 }
 

Simply attach a button component to an empty GameObject

Rendering transparent items is expensive. If the button is to be invisible do away with the image component altogether.

Remember, UI elements are just GameObjects. GameObjects are defined by the attached components. You can combine components in any way you like to produce the behaviour you are after. There is nothing forcing you to stick with the defaults from the Create->UI menu, and in many cases sticking to the default is a bad idea.