How do I introduce a new cat to my existing cat?

The staged introduction over two to six weeks, scent before sight and sight before contact, and the signals that mean go back a step.

Difficulty: intermediate
Time: 1 hr
Read: 6 min
Safety: caution

Short answer

Keep the new cat in its own room completely for the first week and swap scents on cloths and by swapping rooms. Then feed both cats either side of a closed door, moving the bowls closer over days. Only then allow sight through a barrier, and only then brief supervised contact. Expect two to six weeks.

Cats are territorial, and to a resident cat a new arrival is an invasion rather than a friend. The whole method is about letting the two cats become familiar with each other's smell before they ever see each other, and familiar by sight before they can ever reach each other, so there is no single moment when everything changes. Rushing it is the reason so many multi-cat households never settle.

Safety

Never put two unfamiliar cats together and let them 'sort it out'. Cats do not resolve conflict by fighting it out, and a bad first meeting can permanently fix the relationship. Cat bites and scratches to either cat or to you become infected quickly and need veterinary or medical attention. Chronic stress from a badly managed introduction causes real illness in cats — urinary problems, over-grooming and appetite loss — so a slow introduction is a health measure, not just a courtesy.

What you'll need

A separate room with a door that closes
Duplicate litter trays, bowls, beds and scratching posts
A mesh gate or a door wedge
Clean cloths for scent swapping (optional)
Pheromone diffusers (optional)

Introducing a new cat: the sequence over weeks

Step by step

Set the new cat up in its own room before it arrives.Litter tray, food and water well apart from each other, a bed, a scratching post, hiding places and somewhere high. The new cat lives entirely in that room to begin with — it is not confinement, it is a base camp while it works out that this house is safe.
Duplicate every resource for the whole house, plus one.One litter tray per cat plus a spare, multiple feeding stations in different rooms, several water bowls, several beds, several scratching posts and plenty of high places. Almost all cat-to-cat conflict is competition for resources, and duplicating them beforehand prevents most of it.
Days one to seven: scent only, no contact at all.Stroke each cat's cheeks and head with a separate soft cloth and leave that cloth where the other cat rests. Do this daily. Neither cat sees the other. You are aiming for indifference to the cloth rather than hissing at it.
Swap the cats between rooms.Put the resident cat somewhere else and let the new cat explore the house for an hour or two, then swap back. Each cat gets to investigate the other's smell thoroughly without a confrontation. Repeat over several days.
Days seven to fourteen: feed either side of the closed door.A bowl each side, starting far enough back that both eat comfortably, moving them a few centimetres closer each day. This associates the other cat's smell and sound with something good. If either cat stops eating, you have moved too fast — go back to the last distance that worked.
Then allow sight with a solid barrier between them.A tall mesh gate, or the door wedged open a couple of centimetres on a chain. Short sessions of a minute or two, both cats fed or played with, then close it while it is still going well. Several short sessions a day are far better than one long one.
Read what they are telling you.Relaxed sitting, blinking slowly, grooming or eating in each other's sight is progress. A brief hiss is normal and not a failure. Flattened ears, a stare that does not break, a low crouch, a swishing tail, growling, or one cat blocking a doorway means stop the session and go back a step for several days.
Weeks two to four: brief supervised time in the same room.Start with a few minutes with the cats occupied — two people, two wand toys, at opposite ends of the room, or a meal each. End it before anything goes wrong. Build the duration slowly, keeping the pattern that nothing bad ever happens.
Keep escape routes and height available at all times.Neither cat should ever be able to be cornered. High shelves, a cat tree, and doorways left open so either can leave. A cat that knows it can get away is far less likely to fight.
Do not leave them alone together until they are reliably calm.Separate them when you go out, for several weeks. Full acceptance takes two to six weeks in a straightforward case and months in a difficult one, and some cats settle into polite tolerance rather than friendship — which is a perfectly good outcome.

Tips

Two cats that ignore each other have succeeded. Grooming each other and sleeping together is a bonus, not the target.
Pheromone diffusers designed for multi-cat households are worth running throughout the introduction. They are not a substitute for the process but they help take the edge off.
Kittens are generally accepted more readily than adults, and an adult cat introduced to an elderly resident is often the hardest combination. Choosing well at the start does more than any technique later.

Common mistakes

Putting them together on day one to see how it goes — A single bad meeting can permanently set the relationship, and cats do not resolve conflict by fighting. Everything that follows is then remedial.
Not duplicating resources — Competition for trays, food and resting places is the underlying cause of most multi-cat conflict. One tray for two cats guarantees a problem regardless of how well the introduction went.
Pushing on after a hiss because it was 'only one hiss' — The method depends on neither cat rehearsing fear or aggression. Going back a step for a few days costs nothing; pushing through costs weeks.
Comforting or telling off the resident cat for hissing — Hissing is communication. Punishing it teaches the cat that the newcomer's presence predicts something unpleasant from you, which is the opposite of what you want.

If it doesn't work

One cat stops eating during the introduction

Cause: Stress, and a step taken too fast — Fix: Go back two stages immediately. A cat that will not eat for around 24 hours needs a vet, since cats cannot safely go without food.

Resident cat starts spraying indoors

Cause: Territorial stress — Fix: Slow the introduction right down, add resources and vertical space, and see the vet — urinary problems and stress are closely linked in cats.

One cat guards the doorway or the litter tray

Cause: Blocking is a common and easily missed form of bullying — Fix: Add trays in different rooms so no single route can be controlled, and add height so the other cat has alternatives. Reduce contact time and rebuild.

Weeks in and they still hiss on sight

Cause: The process moved too fast at some point — Fix: Go back to complete separation with scent swapping for a week, then rebuild more slowly. A behaviourist referral through your vet is worth it for a stuck introduction.

Questions people ask

How long does it take to introduce two cats?

Two to six weeks for a typical pairing, and months for a difficult one. The pace is set by both cats staying relaxed at each stage rather than by the calendar.

Should I let them fight it out?

No. Cats do not settle disputes that way, injuries from cat fights infect readily, and one bad encounter can fix the relationship permanently. Every stage of the introduction exists to prevent it.

What if they never become friends?

Peaceful coexistence is a perfectly good result and is what many multi-cat households have. With enough duplicated resources and vertical space, two cats that simply avoid each other can both live well.