MORE: 9 Natural Remedies Vets Use On Their Own PetsCALL IN A TRAINERAggression is often fear based, so if there’s no culpable health problem, “engage a trainer to help correct the behavior,” says KC Theisen of the Humane Society of the United States. Positive reinforcement is the way to go.“Once we identify why the dog is aggressing, we plan to treat the underlying cause, not just symptoms. We put boundaries in place and redirect negative behavior into positive. We’ll say no at times, but we don’t shout, hit, or intimidate,” says trainer Victoria Stilwell, host of Animal Planet’s It’s Me or the Dog. “If you get angry, you’ll just fuel the fire.“On one episode, Stilwell used this strategy with Teddy, a Pomeranian that was “fiercely protective” of his female owner, to the point where he’d bite her husband. Using a calm, nonconfrontational manner, Stilwell lifted Teddy off the bed or sofa whenever he growled at the husband’s approach. “Teddy soon learned that the only way to access what he wanted—the wife—was to accept the husband,” Stilwell explains.READ BODY LANGUAGETo contain aggression if it arises again, recognize warning signs. Dogs will usually stand motionless and growl with tails down. Cats can be trickier. “Look for cat body language that signifies anxiety or fear. Signs include flattened ears, dilated pupils, and fur standing on end,” says Jackson Galaxy, the author of Catification and host of Animal Planet’s My Cat from Hell. “Every cat has a different stimulation threshold. Know who your cat is.“Generally, new aggression “is a sign your pet needs you to behave differently,” concludes Theisen. “Most problems are solvable.” MORE: 4 Ways You’re Driving Your Pet Completely Nuts