Some of my cats have dry skin and dandruff. What causes this and how do I treat it?

This may sound stupid, but...how are you sure they have dry skin? Have you seen a vet about this yet? Are there any more details you can give?

One of our cats, Kitty, has noticeable dandruff. My friend says he's always been like that. I asked the vet about it anyway. Well, she said for a black cat he doesn't have much dandruff at all. I stopped worrying ^^

As for dry skin, if that is the problem, what's the humidity in your home?