Because of the nature of the cat, introducing a new cat into a household is often stressful for both the resident cat and the new cat, so this process has to be carried out slowly and carefully. Although cats can obviously be very sociable with other cats, these close relationships only occur when cats recognise each other as part of an established 'social group': cats that are unfamiliar are usually seen as a big threat. This is because the ancestors of our modern cats evolved over millions of years as territorial and largely solitary animals, which needed to defend their territory from other cats in order to catch sufficient prey to survive.
Cats are most likely to form close relationships with other cats where they 'grow up' together, or where the relationship has developed over a long time. Resident cats will be very stressed by the sudden introduction of an unfamiliar cat and may react by being aggressive towards it, hiding away and avoiding the new cat, or showing other signs of stress, such as urine spraying or over-grooming. In fact, problems arising from the stress of introducing the new cat to existing cats are some of the most common reasons for cats being returned to Cats Protection adoption centres after homing. We therefore have to be very careful in the way that we introduce new cats into homes where there are already one or more resident cats.
The important aspect of the process of integration is that it should be slow and gradual. The newcomer should initially be introduced into a room that is not one of the main 'core' areas for the existing cat. This separate room should be set up with a litter tray, food, water bowl, comfy bed, toys and a scratching post. This room is a safe space for your new cat to get used to and to establish as the 'core' part of his territory. It is important for your new cat to relax into his surroundings and get to know you before facing the challenge of meeting your other cat.
Cats rely on scent for communication and identification much more then we do. Because of this, the introduction process starts with the 'swapping' of scent between the cats, for example by exchanging food bowls, toys and bedding. Only when the cats tolerate the scent of the other cat can we gradually progress so that they can see each other. Right through this process, the presence of the other cat is associated with something positive, such as attention, play, or a food treat.
Be prepared, integration can take anything from a few weeks to a few months, depending on the age, character and previous experiences of your cats. Taking your time will make it worthwhile in the end.