Introducing a new cat to the cat you already have

The instinct on bringing a second cat home is to let the two meet and get it over with.

It's also the surest way to start a feud, because to a resident cat a strange one appearing in its space is an intrusion rather than an introduction. The whole method is to let them know each other before they ever see each other.

Set up a base camp before the cat arrives

The newcomer needs its own room from the start: food, water at a distance from the food, a litter tray, somewhere high to sit and somewhere dark to hide. Set it up before the cat is in the house, so the first hour isn't a search for a box.

A door that stays shut is doing the most important work in the whole process, and a sign on it saves the well-meant accident. Everybody in the household needs to know that this door is not one you open absent-mindedly with your hands full.

Expect the newcomer to spend a day or three under something. That's coping, not sulking. Sit in the room and read aloud or work quietly without approaching; a cat that comes out to investigate you on its own terms has taken a bigger step than one that was reached for.

Scent first

To a cat, a familiar smell is a familiar animal. Swap scents deliberately rather than hoping.

Rotate bedding between the two rooms, rub a soft cloth on one cat's cheeks and leave it where the other sleeps, and stroke one then the other without washing your hands in between. Feed both cats on either side of the closed door, so the other's smell arrives at the same time as dinner.

When neither cat reacts to the other's scent — no hissing at the swapped blanket, no bristling at the door — you have permission to move on. That can take three days or three weeks, and the cat sets the timetable.

Site swapping, then sight

Before they see each other, let them explore each other's territory. Shut the resident cat away for an hour and let the newcomer walk the house, then reverse it.

Each learns the other's map without a confrontation, and the newcomer stops treating one room as the whole world. This stage is skipped more often than any other and it's one of the most useful: a cat that has already smelled every corner of the house is much less alarmed by the animal that made those smells.

Then allow seeing without reaching: a door wedged a few centimetres, a mesh panel, a tall baby gate, a glass door. Feed both within sight at a distance where neither stops eating — the distance matters more than the duration, and if one cat abandons its food you're too close. Close the gap over sessions rather than in one afternoon.

First meetings, ended early

Open the door for a few minutes with both cats occupied — food out, a toy going — and end it while everything is still calm.

Ending early is the whole skill. Letting a good meeting run until it goes wrong teaches both cats that the other's company turns bad, and you'll have spent a fortnight of careful work to produce exactly the association you were avoiding.

A little hissing or posturing is normal and doesn't need intervening. Hard staring, flattened ears, a low crouch, a tail thrashing or one cat cornering the other means back up a step.

Never let them fight it out: cats don't resolve conflicts that way, and one real fight can set a relationship back months. If a fight starts, don't reach in — make a loud noise, drop a cushion between them or open an umbrella, and separate them into different rooms until both have settled.

How to tell when to move to the next stage

The commonest failure in this whole process isn't a wrong step; it's a right step taken too early because a week had passed.

The test at every stage is the same and it's behavioural rather than chronological. Is each cat eating normally, using its tray, and behaving as it did before the other arrived? If yes, move on. If either is eating less, hiding more, or on alert at the door, stay where you are for another few days.

Two specific tells are worth watching for. A cat that eats a full meal with the other's scent right beside it has genuinely accepted that scent. And a cat that turns its back on the barrier — sits with the other cat behind it — has decided there's nothing there worth watching, which is a bigger signal than any amount of calm staring.

Most cat conflict is an argument about resources

Two cats sharing one food bowl, one water bowl, one tray and one sunny windowsill aren't two cats living together; they're two cats queueing. Give each its own of everything, in different places rather than side by side, and add height — shelves, a cat tree, the top of a wardrobe — because vertical space multiplies the territory without moving a wall.

Watch the routes as much as the resources. A tray in a dead-end corner that one cat can block is a tray the other stops using, which is one of the commonest ways an introduction shows up as a puddle rather than a fight. Litter box problems goes through the rest of that, and the multi-cat home covers the placement in detail.

What actually predicts how this goes

Age and history do more than anything you control.

Two kittens usually work out fine. A kitten introduced to a tolerant adult is generally straightforward. Two adult cats that both came from single-cat homes is the hard case, and two adult males who have each been the only cat is harder still.

None of it is hopeless; it changes the timeline from a fortnight to a couple of months. What it should change is your expectation rather than your method — the same stages, held longer at each one.

Pheromone diffusers are widely sold for this and the evidence for them is mixed rather than strong. They're unlikely to do harm and they're not a substitute for the process above, so treat one as a small possible help rather than the plan.

Choosing the second cat

Worth a moment, because the choice is made before any of the above applies and it matters more than most of it.

A confident, playful young cat and an elderly one who has been alone for a decade is a difficult pairing whatever you do, because the mismatch is about energy rather than goodwill: the young cat's invitation to play is, to the old one, harassment.

Matching roughly on age and activity level is a better predictor of a quiet house than matching on sex, colour or breed, none of which tell you much. If you're adopting, a rescue that has fostered the cat in a multi-cat home can tell you something genuinely useful about how it behaves with others — ask, because it's the single most informative thing available.

Introducing a cat to a kitten, in reverse

One asymmetry worth planning for: the resident cat's reaction to a kitten is usually milder, and the kitten's needs are greater.

Kittens have no sense of a stranger's personal space and will approach an adult cat that has clearly asked to be left alone. That produces a swat, which is a perfectly appropriate piece of teaching from the adult and looks alarming to the owner. Let it happen, provided the adult can get away and the kitten isn't being pursued.

What the adult needs is an exit and a high place the kitten can't reach yet. Give it those and most adult cats do the job of raising a kitten's manners far better than any human intervention.

When it does not become friendship

Plenty of cats never groom each other and never share a bed, and that's a perfectly successful outcome.

The realistic goal is peaceful coexistence: no fighting, no one cat blocked from food or the tray, no one cat living under a bed. Measured against that, a great many introductions people describe as having failed actually worked.

If after a couple of months there's still real aggression, or one cat has stopped eating, is hiding constantly or urinating outside the tray, that's the point to involve a vet — stress makes cats physically ill — and then a behaviourist, rather than waiting for time to fix it. Why cats fight covers the way back when a relationship has properly broken down.