It's simple and painless -- and best for indoor dogs that often go collarless. Your vet injects a tiny computer chip (the size of a grain of rice) under the animal's skin between the shoulder blades. Along with your contact info, the chip's number is added to a database accessible to animal hospitals, shelters, and humane societies. Since the chip can't get lost or damaged, it lasts for a pet's lifetime. Ask your vet for info or go to homeagainid.com.
Pros: An affordable, permanent way to identify your pet.
Cons: The procedure must be performed by a vet. Plus, the animal, once found, has to be picked up by a vet or shelter with an ID scanner.
Cost: $25 to $40