Introducing a new cat to a household with an existing cat (or cats) is one of the areas where rushing the process most reliably backfires — a bad first impression between two cats can take months to undo, while a properly gradual introduction, even though it feels slow at the time, often reaches a comfortable coexistence faster overall.
Separate Spaces First
The new cat should have its own separate room with its own litter box, food, water, and bedding for the first several days to a week or more, with no direct visual contact with the resident cat(s) yet. This gives the new cat a secure base and lets both cats begin adjusting to each other's scent (through the door, and via swapped bedding) before any visual or physical introduction.
Scent Swapping
Before any visual introduction, swapping bedding or gently rubbing a cloth on each cat and placing it near the other's space lets both cats become familiar with each other's scent in a low-stakes way. Feeding both cats near the (closed) door that separates them, gradually moving the food bowls closer to the door over several days, is a commonly recommended technique to build a positive association with the other cat's scent.
Visual Introduction Without Full Access
A cracked door, a baby gate, or a screen door between rooms allows visual contact while preventing full physical access — this stage should be brief at first and gradually extended, watching for relaxed body language (normal ear position, no prolonged staring or growling) before progressing.
Supervised Direct Introduction
Only once the above stages are going reasonably smoothly should a brief, supervised direct introduction happen — ideally during a calm time, with an easy way for either cat to retreat, and kept short at first. Some hissing or posturing during early direct meetings isn't necessarily a failure sign, but sustained aggression, chasing, or one cat hiding and refusing to come out are signs to slow back down to an earlier stage rather than pushing forward.
Realistic Timelines
A full, comfortable integration can reasonably take anywhere from a few weeks to a couple of months, and sometimes longer for particularly territorial cats — this is normal, not a sign the introduction is failing, as long as things are trending in the right direction even slowly.
When to Get Help
If aggression persists or escalates despite a properly gradual approach, a consultation with a feline behavior specialist is a reasonable step before assuming the cats simply "can't" get along — many seemingly incompatible pairings do eventually settle with the right approach and enough time.