The instinct on bringing a second cat home is to let the two meet and get it over with. It is also the surest way to start a feud, because to a resident cat a strange one appearing in its space is an intrusion, not an introduction. The trick is to let them know each other before they ever see each other.
Scent comes first
Give the newcomer its own room to begin with — food, water, a litter tray and somewhere to hide — and keep the two apart. Let them learn each other through the door, and swap their scents deliberately: trade bedding between the rooms, or stroke one and then the other, so each grows used to the smell of the other before a whisker is seen. To cats, a familiar scent is halfway to a familiar cat.
Sight before contact
Once neither reacts to the other's smell, allow them to see without reaching — a door cracked open, or a mesh barrier. Feeding both cats within sight of each other, at a distance neither finds alarming, quietly teaches that the other's presence comes with something good. Close the gap over several sessions rather than in one.
Let the cats set the pace
First full meetings should be short, supervised and ended while things are still calm. A little hissing or posturing is normal; steady staring, flattened ears or a chase means back up a step and go slower. Give each cat its own food, water, litter and high perches, so neither has to compete for the basics — much conflict is really an argument over resources. If real hostility persists for weeks despite a patient approach, a vet or feline behaviourist can help rather than leaving them to sort it out.
How long does it take to introduce two cats?
Anywhere from a few days to a few weeks, depending on the cats. Going at the slower cat's pace — scent, then sight, then supervised meetings — beats rushing, which tends to set the relationship back further than it saves.
Should I let my cats fight it out?
No. Letting cats work it out through conflict tends to harden a bad relationship rather than settle it. Separate them, drop back a stage, and rebuild more slowly; persistent aggression is worth a behaviourist's help.
Why does my resident cat hiss at the new one?
Because a strange cat in its territory is a threat, not a friend, until proven otherwise. Some hissing early on is normal; the scent-first, sight-next approach gives the resident cat time to reclassify the newcomer as safe.