Probably a basic question

This is probably a really stupid question but here goes. If my project utilises 12v, 5v and 3.3v, can the different voltages share the same -ve / earth?

Not only can they, they almost always have to.

Delta_G: Not only can they, they almost always have to.

It though so, I am making use of a transistor to get the 5v signal from the arduino to power a piezo sound bomb that is 12v. An example of wiring a transistor online seems to show shared earth. Thanks