Slow introductions prevent lifelong friction. Start by giving the newcomer a safe room with food, water, a litter box or crate, and hiding spots. Let resident pets investigate the scent under the door before any face-to-face meeting.
Scent swapping is the secret weapon. Trade bedding between animals so they learn each other's smell without threat. Feed both pets on opposite sides of a closed door so positive experiences (food) become associated with the other's presence.
For dog-and-dog meetings, use neutral territory like a quiet park, keep both on leashes, and reward calm behavior. For cat-and-cat, use a cracked door or a baby gate so they can see but not touch, gradually increasing exposure over days.
Dog-and-cat introductions need supervision for weeks. Teach the dog that the cat is not prey by rewarding focus on you. Give the cat vertical escape routes — cat trees and shelves — so it never feels trapped.
Progress at the slowest animal's pace. Rushing causes fear that can harden into aggression. Most households reach peaceful coexistence within a few weeks when the process is patient and reward-led.