Cats can exhibit territorial aggression toward people or other cats that approach or live on their property.  This can happen when a new cat is added to the household, when resident cats reach social maturity, when one cat is removed from the household for a short period and then brought back into the home, or when new outside cats come into the territory.

Behaviors often seen in territorial aggression include stalking, chasing, and aggressive encounters, which can lead to injury.  At times, the aggressive cat will prevent the victim from having access to certain areas of the home, resulting in a cat that lives on top of furniture and bookshelves or under beds.  This can be related to the "social status" of each cat, as well as lead to other problems such as house soiling from not using the litterbox.  Although a cat may act aggressively toward another cat because of territoriality, other components of fear, sociability, and social status can also contribute to the aggressive response.

Territorial aggression can be prevented or minimized with early socialization, patient and slow introductions of new cats, and adequate resources, including multiple litter boxes and food bowls.  There should also be sufficient space for climbing and hiding, and for each cat to be able to distance itself from others when it chooses.  When introducing (or reintroducing) a new cat into a household with existing cats, problems can best be prevented by slowly introducing the new cat to the environment, by initially keeping the new cat in a separate room with water and a litterbox, and by supervising all interactions.

The correct time to begin cat-to-cat interactions can be highly variable.  If both cats have been adequately socialized with other cats and are not too timid or fearful, it may be only a few days or weeks before the cats work out their social structure on their own.  However, in some cases, the aggression between cats persists.  In these situations, a lengthy separation may be required in addition to a more formal desensitization and counter-conditioning program.

Despite slow and careful progression, some cats may continue to display aggression, and it may be necessary to accept that they may never be compatible housemates.  Although cats do live in social groups, they also can choose to leave a particular group.  The social groups we create in our homes do not provide that opportunity.  The only way to avoid territorial competition in these cats may be to find a new home for one of the cats, or to provide separate living quarters for each cat within the home.