You have created a very complex illustration with a lot of objects and numerous layers. As the illustration moves along to completion, you decide to tidy up a bit. Some of the layers only have a few objects in them and really do not need to be separate layers. So you decide to combine some of these layers. Is there an easy way to do this?
Yes! In the Layers palette (Window > Layers), click on one of the layers to make it the Active Layer. Then Command-Click (Control-Click on PC) the remaining layers that you would like to combine with the first layer. Make sure to click last on your destination layer for all the objects from the layers. Go to the Options flyout menu of the Layers palette and select Merge Layers. All the objects will move to the destination layer maintaining their original stacking order.
Tip provided by Jeff Witchel, Certified Adobe Training Provider.