There are many different types of aggression in dogs. You deal with each kind differently. Its very important to know what kind of aggression your dogs has before you do anything. For example, if your dog has fear aggression but you deal with the issue as if he had dominance aggression, you would only make it worse.
Try this link for a summary of the different types.
http://www.unclematty.com/training/aggressiontypes.htm#