The dog, Canis familiaris, of the dog family is thought to have descended from a wolf-like creature that lived millions of years ago. History suggests that the dog was one of the earliest of tamed animals, having lived in close with humans for about 10,000 years. Following domestication, dogs were selectively bred for specific purposes. The earliest remains of domestic dogs date from the late Middle Stone Age. These early dogs resembled the dingo, a wild dog native to Australia. Because dogs often tend to be scavengers, it has been suggested that they adopted humans rather than the other way around.

Although dogs have a lot based features, and members of a single litter may be born with marked mixes, the variation in size, shape, and color of domesticated dogs is almost entirely the result of artificial breeding and selection. The American Kennel Club (AKC) recognizes more than 140 breeds, grouping them as sporting dogs, hounds, working dogs, terriers, toys, non-sporting dogs, and herding dogs. The AKC registers more than 1.3 million purebred puppies per year. Among the most popular breeds in recent years are Labrador retrievers, Rottweilers, German shepherds, golden retrievers, poodles, and cocker spaniels. AKC registration indicates only that a dog's parents are registered purebreds; it is not an indication of quality. Mixed-breed dogs are not eligible for registry. The British Kennel Club lists more than 210 breeds, with about 70 member countries, lists more than 400 breeds. In the United States the number of dogs, both purebred and mixed breed, is estimated to range up to 30 million.

The dog has been bred for many domestic purposes other than as pets. Keen senses of hearing and smell have made dogs valuable in hunting and tracking and for guard, security, and search purposes. Dogs also herd livestock and serve as guide dogs for the blind, as sled dogs, and as racing dogs. Their positive effect on people is well proved; they have been used in programs for treating the mentally ill.

Dogs vary more in outward looks than the inside structure. An adult dog may weigh 2 to 99 kg (4 to 220 lb), depending on the breed, and range in height from 12.5 to 90 cm (5 to 35 in) at the shoulder. Other differences in form include length of leg; length of muzzle; size and attitude of ears; length, shape, and carriage of tail; and length, thickness, color, and character of hair. Over time, the various breeds were also selectively bred to produce characters suited to the tasks they performed.

The female dog reaches reproductive maturity between 7 and 18 months of age. The pregnancy period averages 62 to 63 days. Sightless at birth, puppies do not open their eyes until about the tenth day. They are generally weaned by the sixth week. A dog is full grown at age 2, is old at 12, and seldom lives beyond the age of 20.

A dog's most important sense is smell. Dogs are unable to differentiate colors ranging from greenish yellow through orange to red but can distinguish between colors at opposing ends of the spectrum. An adult dog has 42 teeth, including molars. Dogs do not perspire; heat is removed through evaporation from the nose and footpads and from the tongue and respiratory tract (panting). The dog has a simple carnivore stomach but eats cooked vegetables and grain as well as meat.

Before choosing a dog, consider your life-style. Some breeds tend to be noisier, more active, or better with children than others; some require a great deal of exercise or grooming. Although both mixed breeds and purebreds make good pets, it is easier to predict the adult physical characteristics and temperament of a purebred puppy. One way to become acquainted with various breeds is to attend an all-breed dog show. Some breeds suffer from serious hereditary defects for which the parents of your puppy should have been tested. Most responsible breeders also will insist that you spay or neuter a pet quality puppy.

A dog should be checked regularly by a veterinarian to ensure that it remains in good health. Puppies are given a series of shots between about 6 and 16 weeks to protect them against a variety of infectious diseases, including distemper, hepatitis, leptospirosis, parvovirus, coronavirus, and parainfluenza ("kennel cough"); annual boosters against these diseases are necessary to maintain immunity. Dogs also need to be protected against rabies and checked for intestinal parasites. The presence of such parasites is usually indicated by a failure to thrive despite adequate nutrition and by occasional vomiting and diarrhea. Another serious internal parasite, heartworm, is transmitted by infected mosquitoes and can be protected against by giving oral medication during mosquito season. The most common external parasites affecting dogs are the flea and the tick. In cases of severe infestation, both the dog and the premises must be treated.

A well-trained dog is a delightful companion. Consistency is critical in dog training, as is generous praise when the dog behaves in the proper manner.