Before tackling your cat’s aggressive behavior, discuss your concerns with a veterinarian. Some aggressive behavior can indicate illness.

AGGRESSION WHILE PLAYING: Kittens normally replace most of their social behavior with rough play at about 12 weeks of age. Young adult cats again become aggressive to each other around 8 or 9 months of age. Redirect play to appropriate toys or walk away. 

PREDATORY BEHAVIOR: Cats are predators. Much of their behavior is suited to catching food they need to survive and resting up for the hunt. In our homes, where we provide food, your cat may attempt to burn off extra energy by "pouncing", biting and scratching. Punishment will not stop this activity because you cannot always be present when the cat is hunting and the activity is satisfying. In some cases, your punishment becomes part of the game.

If your cat is stalking and chasing your feet, keep a spray bottle of water or compressed air handy and spray him when he attacks. Tossing an appropriately weighted bean bag at your cat right before he pounces is also a good distraction. If he is persistent, offer him catnip in a closed room with appropriate toys and leave the room. Keep a few bath towels handy to toss over him so you can move away safely when he is charged up. 

Keep your cat from storing up energy by hiding treats around the house in areas where you want to encourage your cat to play such as around his scratching post. Offer a cat nip filled sock, a ping pong ball in the dry bathtub (kitty racquet ball), or paper sacks (no shopping handles) to encourage play and exercise. Some cats enjoy chasing a flashlight beam or motorized toy. You can buy fishing pole toys so that you can be part of the games. 

HORMONAL AGGRESSION: Some aggressive behavior is hormonally driven. In these cases, neutering is a necessary part of the solution. Some female cats are aggressive when they are in heat and some intact cats are aggressive when they are exhibited at shows. If your cat can not be neutered you may consider medication from your veterinarian that can be used to calm your cat during these infrequent episodes. If your pet has an aggressive behavior problem, he or she may not be suitable for breeding anyway because of the risk of producing offspring with the same aggressive behavior.

AGGRESSION TOWARD STRAY CATS: Contact with stray cats can make your cat sick. Feline Immunodeficiency, leukemia and rabies are deadly. Cat repellents may help but they also offend your own cat. You can buy a motion sensing water sprinkler called a Scare Crow or install cat fencing. You can build an outdoor enclosure for your cat or consider making your cat an indoor cat. 

AGGRESSION TO A NEW PET: If your cat is not accepting a new member of the household, use the procedure described for introducing a new cat or new dog to your household. 

DISPLACED AGGRESSION TOWARD A COMPANION: If aggression has occurred suddenly between cats who previously got along, it is likely to be related to a specific incident such as one cat returning home after an absence or one cat being injured or startled and finding the other cat nearby. Cats will sometimes turn on each other because an intruder is out of reach. Cats are not rational when they are threatened. Reprimanding an attacking cat doesn't work. In these cases you can help restore peace by separating the cats overnight. Rub each with a towel in turn so that they become re acquainted with each other's scent while they are relaxed. Allow them to come across each other normally the following day. A recommendation to place a drop vanilla extract on each cat may work. It causes the cats to focus on a new scent which they both share and to spend time grooming which is a calming behavior.

OVER STIMULATION WHILE PETTING THE CAT: If your cat's aggression comes about suddenly while you are petting him, it may be caused by sensitivity, especially at the base of his tail. Stroking your cat repeatedly from head to tail is most likely to cause over stimulation. If your cat suddenly puts one or both ears against his head while being stroked, gently stand so that your cat drops onto the floor and return to your seated position to avoid passing your cat while he is still charged up. You can still enjoy petting your cat but replace stroking from head to tail with scratching his head and neck only.

AGGRESSION WHILE RESTRAINED: If your cat is aggressive to you when you are giving medication or clipping nails, you should wrap him in a towel to do these things. To prevent your cat from fearing the towel, you can wrap him in a towel for about 5 seconds, a few times each week. A groomer or veterinary technician can trim his nails for you, if he becomes too upset when you restrain him at home. 

If your cat is unpredictably aggressive or people are being injured by your cat you should speak to a veterinarian and behaviorist about formal training and even medication for your cat.