If it's on concrete and firmly glued down, it shouldn't be a problem to just install over it. If you're installing over vinyl attached to a sub floor, that requires a little more work. You need to screw down a half inch concrete board before tiling. Make sure you use screws that won't rust. If that makes the floor too high, they do sell some thinner material that's just as strong. You just need to make sure the floor doesn't have bounce or you'll end up with cracked tile or mortar.