Instructions
Get all cats in your household spayed or neutered to naturally lower their aggressive tendencies.
Place a new cat in an isolated room that's fully equipped with food bowls, bedding, toys, a scratching post and a litter box. Allow your other cats to come near the door, but leave it closed.
Feed the cats on opposite sides of the door. This way, they can smell each other but not see each other. It also makes them associate the presence of the other cat or cats with feeding time, which is a pleasant experience.
Place your existing cats in the new cat's room and vice versa. This gives them a chance to get to know each other's smells. Once you switch back, exchange their bedding and scratching posts so some of the smell from the other cat or cats remains in their existing environment.
Place one person on either side of the door that separates the cats after about a week. Replace the door with a screen or baby gate. This allows the cats to see each other. Give treats as soon as they notice the other cat or cats. Encourage them to play between the barrier and continue giving treats. Do not leave the barrier unsupervised. Close the door once you leave the room.
Introduce the new cat to each of your existing cats face-to-face one at a time. Do not bring the new cat into a room with multiple existing cats. Wait until after a meal to introduce them so they're calm. Allow the cats to stay in the same room for several minutes and give treats when they're calm. Do this with each cat. Over the next week or two, increase the length of time the new cat is around each existing cat. Once they all seem to get along for a long period, you can leave them unsupervised.
Bring an overly aggressive cat to your veterinarian. It may have an underlying health issue that's causing the aggression. Once it's treated, the cats should all get along.
Introduce the cats again. If your cats are still not getting along, you may have rushed through the introduction phase. Start the introductions again by separating the cats. Spend more time on each phase than you did the first time to help teach the cats to get along.
How to Have All Cats in the House Get Along
When you have multiple cats in your household, they sometimes don't get along. In some cases, cat aggression and bad behavior toward one another results when you improperly introduce them to each other. When you get a new cat, take the time to make the introductions to your existing cats the right way to prevent future behavioral problems.