Done well, most dogs adjust within a few weeks. Done poorly, you can create lasting tension.
Choose a Compatible Dog
Before you commit, consider whether this dog is a good match for your current dog.
Size: A very large dog may overwhelm a smaller one.
Energy level: A sedate older dog won't enjoy a hyperactive puppy. A high-energy dog may frustrate a mellow companion.
Temperament: An assertive, outgoing dog may make a shy dog retreat. Two dominant dogs may clash.
If you're adopting, bring your resident dog to meet the candidate before deciding. Your dog is going to spend more time with this newcomer than you will.
Introduce on Neutral Territory
Don't bring the new dog straight into your home. Your resident dog considers the house, yard, and even the block "theirs."
Meet in a neutral location--a quiet park (not a dog park), a friend's yard, or another area neither dog has claimed. Keep both dogs on leash with some slack. Let them sniff and greet briefly.
Watch for stress signals: hackles raised, stiff posture, hard stares, growling, or lip curls. If either dog shows aggression, end the meeting and reconsider whether this match will work.
If things go well--relaxed body language, play bows, or polite disinterest--let them interact for a few minutes, then separate and give each dog a treat. This creates a positive association with the other dog's presence.
The First Days at Home
When you bring the new dog home, expect some tension. Your resident dog's world has been disrupted.
Feed separately. Food is a common trigger for conflict. Use different rooms or crates until you understand how both dogs behave around resources.
Supervise all interactions. Don't leave the dogs alone together until you're confident they're comfortable. This may take days or weeks.
Give your resident dog breaks. Puppies especially are relentless. They don't understand personal space. Your older dog needs time away from the newcomer--a room of their own, walks without the new dog, quiet time with you.
Introduce new toys. Don't expect dogs to share the resident dog's possessions. Bring in fresh toys that neither dog has claimed.
Puppies and Older Dogs
Older dogs generally give puppies a "puppy pass"--they tolerate rude behavior that they wouldn't accept from an adult dog. But this patience has limits.
If your puppy is pestering your older dog relentlessly, intervene before your older dog has to correct the puppy harshly. Crate the puppy for naps. Give your older dog sanctuary.
The goal is to keep early interactions positive. A puppy who learns that the older dog predicts good things (calm greetings, shared walks, treats) will develop respect. A puppy who only experiences corrections will become anxious or defensive around the older dog.
Why This Takes Time
Dogs don't become friends instantly. They're figuring out how to share space, resources, and your attention. Some dogs bond quickly; others need months to settle into a comfortable coexistence.
The household hierarchy will recalibrate. There may be minor squabbles as the dogs sort out who defers to whom in various contexts. This is normal unless it escalates to actual aggression.
Your job is to manage the environment, prevent conflicts over resources, and give both dogs positive experiences together. The relationship will develop on its own timeline.