What is the difference between T-12 and T-8 shop light bulbs? Can the new T-5 bulbs be used in my old shop light fixtures?

To my knowlege you can use them if they fit. I think the many different ballasts employed today have different sockets if voltage of ballast differs. just look closley before twisting one in. good luck...

The difference is the diameter of the tube. You can NOT use them interchangeably. The ballast has to be rated to the type of tube installed. It also has to be rated to size of the tube. They come in lengths from 2 feet to 8 feet. T12 is 12/8 or 1.5 diameter T8 is 8/8 or 1 inch diameter T5 is 5/8 inch diameter.