Even though all of my cats are indoor only, I've been using Advantage on them every month for many years. When I didn't, they would get fleas. I live in a condo and there are other animals close by. If they have fleas then my cats will get them too. I have to use it year round because the weather is very mild here in CA. I also buy the largest dog advantage and then I measure it for my cats. This saves me a lot of money and it's been preapproved by my vet. In fact one of my former vets was the one who recommended doing this to help save me money.