My cat Sergei is a biter. Have your cats always done this? There could be various reasons. For my cat, he seems to bite the most when he is playing or overstimulated. Some cats bite to get attention, love bites, or simply to be left alone, for instance during a "petting session", the cat may be contentedly purring then WHAM! He/she turns around and bites ya. In that instance, it's overstimulation which is the cat's way of saying "ok, that's enough". Do your cats typically bite or nip you during play or petting? Like Wayne mentioned, it should be a pretty easy thing to get them to stop. I've only had Sergei for a short time, but he has greatly improved with the biting behavior. Good luck!