Cats are meant to be outdoors; it's their natural habitat. When I was growing up, the only way I was allowed to have a cat was if I let it run outdoors. And they all died early deaths, some of them not even living to be a year old. Too much can happen to a cat; cars, kids, airborn illnesses, etc. So unless you're willing to see your cats die young and possibly horrible deaths, keep them indoors.