Like most predatory mammals, the dog has powerful muscles, a cardiovascular system that supports both sprinting and endurance, and teeth for catching, holding, and tearing.

The dog's ancestral skeleton provided the ability to run and leap. Their legs are designed to propel them forward rapidly, leaping as necessary, to chase and overcome prey. Consequently, they have small, tight feet, walking on their front toes; their rear legs are fairly rigid and sturdy; the front legs are loose and flexible, with only muscle attaching them to the torso.

Although selective breeding has changed the appearance of many breeds, all dogs retain the basic ingredients from their distant ancestors. Dogs have disconnected shoulder bones (absent the collar bone of the human skeleton) that allow a greater stride length for running and leaping. They walk on four toes, front and back, and have vestigial dewclaws (dog thumbs) on their front and rear legs just like monkeys thumbs, in some cases these claws are missing due to surgery. The rear dewclaws are sometimes removed to prevent the possibility of them being ripped off, or catching on something and breaking, especially in dogs with loose dewclaws. This practice is illegal in some countries but not in others.

The dog's ancestor was about the size of a Dingo, and its skeleton took about 10 months to mature. Today's toy breeds have skeletons that mature in only a few months, while giant breeds such as the Mastiffs take 16 to 18 months for the skeleton to mature. Dwarfism has affected the proportions of some breeds' skeleton, as in the Basset Hound.

Modern dog breeds show more variation in size, appearance, and behavior than any other domestic animal. Within the range of extremes, dogs generally share attributes with their wild ancestors, the wolves. Dogs are predators and scavengers, possessing sharp teeth and strong jaws for attacking, holding, and tearing their food.

Dogs are susceptible to various diseases, ailments, and poisons, some of which affect humans in the same way, others of which are unique to dogs.

Molecular systematics indicate that the domestic dog is descended from a wolf-like ancestor. As reflected in the nomenclature, dogs are a subspecies of wolf and are thus still able to interbreed.

The relationship between man and canine is a long-standing tradition. Wolf remains have been found in association with hominid remains dating from 400,000 years ago. Dogs were, and are, valued for their aid in hunting. Dog burials at the Mesolithic cemetery of Svaerdborg in Denmark indicate that in ancient Europe dogs were valued companions. The molecular genetic data suggests that the domestic lineage separated from modern wolves around 150,000 years ago (Vilà et al, 1997). Conservative estimates propose that the domestication of the dog probably occurred at least 14,000 years ago. There is archaeological evidence of dog remains that are morphologically different from wolves from that time frame. However, in the early 2000s some research indicated that domestication in fact had already begun to occur as early as 100,000 years ago.

Relationships between humans and dogs are often characterized by strong emotional bonds. Consequently, dogs are popular as [pet]s and companions, independent of any utilitarian considerations. Many dog owners consider having unconditional acceptance from a friend who is always happy to see them to be quite utilitarian, particularly if the dog also leads them to regular exercise. Empirically, dogs are quite dependent on human companionship and may suffer poor health in its absence. Many dogs are reported to have separation anxiety if its owner is away for an extended period of time.

Some research demonstrates that dogs are able to convey a depth of emotion not seen to the same extent in any other animal; this is purportedly due to their closely-knit development with modern man, and the survival-benefits of such communication as dogs became more dependent on humans for sustenance.

Nevertheless, it is often unwise to anthropomorphize the responses of dogs. Despite understandably positive interpretations by dog owners, it is questionable whether these animals are truly capable of feeling emotions on a human level. More research is needed to determine the intelligence level of dogs, and the motivations behind their responses to their masters.