Painting - painting cabinets

I'm about to start painting my kitchen cabinets and I'm not sure if I should sand or prime them first. There are so many old layers of paint underneath that they are all peeling off, and I doubt another coat will stick very well.
What is they best way to prime them?

If some of the old layers of paint are coming off, then you must take care of that problem first. Sorry but this is a lot of work. The adhesion of a coat of paint will be as good as the adhesion of the surface on which it is painted. You can either scrape off all of the paint that is loose and then paint or you can use a stripper and strip all of the paint down to the original wood surface or one of the coats of paint that has good adhesion and then paint. If you strip down to the original wood surface, use a latex primer and a latex topcoat. If you want a real high gloss finish, use an oil based primer and topcoat. Using a stripper is messy but sometimes it is very worth while because eventually it will provide you with a smooth surface to paint.