The Chihuahua is the smallest breed of dog and is named for the Chihuahua region in Mexico. The standard recognized by the AKC (American Kennel Club) is only known as "Chihuahua", but they come in many varieties of morphology and colors with either long or smooth coats. Their weight may range from 2 to 10 lb (1 to 5 kg), although the AKC disqualifies dogs over 6 lb (3 kg), and they have distinctive large ears and tails that stand straight up.

Prized for their personality and loyalty, they are thought to be descendants of an ancient, similar, but slightly larger breed associated with royalty in Aztec civilizations known as the Techichi. They are the oldest canine breed in North America.

Because of their size and physical fragility, they are not well-suited as children's pets and require expert veterinary attention in areas such as birthing and dental care. They are also prone to some genetic anomalies, often neurological ones, such as seizure disorders and patella luxation. They are also known for their moleras. However, their loyalty, alertness, intelligence and size makes them easily adaptable to a variety of environments including the city and small apartments and make for usually long lifetimes of 15 years or more of loving companionship.