Adding a new puppy or kitten is exciting, but it can also be stressful for both the resident pets and people. The most important thing to remember when introducing a new pet is to take things slow, but there are other steps that can be taken to make the introduction as smooth as possible.

In general, dogs tend to be more open to the addition of new pets than are cats, because dogs are pack animals by nature. However, problems like personality conflicts are possible. It is best to confine the new pet to a crate for the initial introduction. This allows the resident pet(s) to meet the newcomer without risk of injury to either pet. This will also reassure the older pets that the newcomer is not going to immediately try to take over their social position. If the initial greeting is friendly, they can then be allowed to interact with one another while both are kept on a leash (so they can be quickly separated if necessary). If things are going smoothly, they can be allowed more contact and play sessions over the next several days. Mealtimes should be supervised for at least several weeks, not only to keep them from stealing each other's food, but also because food is one of the more common triggers for dog fights.

If there is a large difference in age and activity levels between the dogs (for example, when introducing a new puppy to an older dog), play times will probably need to be supervised and limited for several months. A 12 year old Labrador may not have much patience with a 2 month old puppy wanting to run around, chewing on its ears, pounce, etc. When you notice the older dog trying to get away from the more active one, they should be separated for several hours, or it may end up biting the younger one to correct its exuberant behavior.

Since cats tend to be territorial, it can be more difficult to get them to warm up to a newcomer. In the beginning, the new cat or kitten should be confined to one room with its own litter box, food and water. The cats should be able to smell and hear each other but no contact should be allowed. After a few days, switch the positions of the cats so the new one is allowed to explore the house, while the resident cats can investigate the smell of the newcomer. Switch them back after they have had time to explore.

The next step is to place them on opposite sides of the same room, either in carriers or restrained with harnesses and leashes. Both cats should be fed, so they learn to associate the pleasure of eating with each other's presence. If the cats won't eat or seem anxious or aggressive, they are probably too close together. However, if they eat and seem relaxed, they can be moved closer together at the next feeding session. In between sessions, the new cat should still be confined to its own room. The final step is to release them in the same room and feed them, still keeping them far apart. Monitor them closely for any aggression or anxiety; this indicates that the introductions are proceeding too quickly.

For dogs or cats, the entire process can proceed only as quickly as the pets allow, and can sometimes take weeks or even months, especially with cats. However, taking the time to introduce pets properly is invaluable to preventing behavior problems over time.