Pervasive in human mythology, folklore, and language, the gray wolf has had an impact on the human imagination and been the victim of levels of misunderstanding that few animals have shared. With the exception of humans and the lion, the gray wolf once had a larger distribution than any other land mammal, once ranging over all of North America from Alaska and Arctic Canada southward to central Mexico and throughout Europe and Asia above 20° N latitude. It lived in every type of habitat except tropical forests and the most arid deserts, and it was the premier hunter of the large hoofed mammals. Approximately 5 subspecies are recognized in North America, 7 to 12 in Eurasia. Wolves were domesticated several thousand years ago, and selective breeding produced dogs.

The wolf is built for travel. Its long legs, large feet, and deep but narrow chest suit it well for life on the move. Keen senses, large canine teeth, powerful jaws, and the ability to pursue prey at 60 km (37 miles) per hour equip the wolf well for a predatory way of life. A typical northern male may be about 2 metres (6.6 feet) long, including the bushy half-metre-long tail. Standing 76 cm (30 inches) tall at the shoulder, it weighs about 45 kg (100 pounds), but weight ranges from 14 to 65 kg (31 to 143 pounds), depending on the geographic area. Females average about 20 percent smaller than males. The largest wolves are found in west-central Canada, Alaska, and across northern Asia. The smallest tend to be near the southern end of their distribution (the Middle East, Arabia, and India). Fur on the upper body, though usually gray, may be brown, reddish, black, or whitish, while the underparts and legs are usually yellow-white. Light-coloured wolves are common in Arctic regions.

Early human societies that hunted for survival admired the wolf and tried to imitate its habits, but in recent centuries the wolf has been widely viewed as an evil creature, a danger to humans (especially in Eurasia), a competitor for big game animals, and a threat to livestock. Depredation of livestock was the primary justification for eradicating the wolf from virtually all of the United States, Mexico, and most of Europe. Wolves in the United States were killed by every method imaginable in the 19th and early 20th centuries, and by 1950 they remained only in the northeastern corner of Minnesota. In the late 20th century, greater tolerance, legal protection, and other factors allowed their range to expand in portions of North America and Europe.

Wolves are probably more popular now than at any other time in recorded history. In 1995 wolves from Canada were reintroduced to Yellowstone National Park and Idaho, and captive-reared Mexican wolves (a subspecies) were released to their former range in eastern Arizona beginning in 1998. At the beginning of the 21st century, an estimated 65,000–78,000 wolves inhabited North America. Canada had the largest population (although the provinces of New Brunswick, Nova Scotia, and Prince Edward Island had no wolves), followed by Alaska and Minnesota. Some of the western states as well as Michigan and Wisconsin have smaller but recovering wolf populations. Canadian wolves are protected only within provincial parks, whereas all wolves in the contiguous United States receive some level of legal protection by federal and state governments. Populations in southern Europe and Scandinavia are relatively small but are increasing. The Eurasian population probably exceeds 150,000 and is stable or increasing in most countries, and most afford the wolf some degree of legal protection. Worldwide, wolves still occupy about two-thirds of their former range. Although often thought of as wilderness animals, wolves can and do thrive close to people when they are not excessively persecuted and food is available.

Wolves usually live in packs of up to two dozen individuals, but packs numbering 6 to 10 are most common. A pack is basically a family group consisting of an adult breeding pair (the alpha male and alpha female) and their offspring of various ages. Each individual has its own distinct personality. The ability of wolves to form strong social bonds with one another is what makes the wolf pack possible. A dominance hierarchy is established within the pack, which helps maintain order. The alpha male and female continually assert themselves over their subordinates, and they guide the activities of the group. The female predominates in roles such as care and defense of pups, whereas the male predominates in foraging and food provisioning and in travels associated with those activities. Both sexes are very active in attacking and killing prey, but during the summer hunts are often conducted alone.

A pack's territory can be 80 to 3,000 square km (31 to 1,200 square miles), depending on prey abundance, and it is vigorously defended against neighbouring packs. Wolves communicate with one another by visual signaling (facial expression, body position, tail position), vocalizations (vocalization), and scent marking. Howling helps the pack stay in contact and also seems to strengthen social bonds among pack members. Along with howling, marking of territory with urine and feces lets neighbouring packs know they should not intrude. Intruders are often killed by resident packs, yet in some circumstances they are accepted.

Breeding occurs between February and April, and a litter of usually five or six pups is born in the spring after a gestation period of about two months. The young are usually born in a den consisting of a natural hole or a burrow, often in a hillside. A rock crevice, hollow log, overturned stump, or abandoned beaver lodge may be used as a den, and even a depression beneath the lower branches of a conifer will sometimes suffice. All members of the pack care solicitously for the young. After being weaned from their mother's milk at six to nine weeks, they are fed a diet of regurgitated meat. Throughout spring and summer the pups are the centre of attention as well as the geographic focus of the pack's activities. After a few weeks pups are usually moved from the den to a “rendezvous site” above ground where they play and sleep while adults hunt. The pups grow rapidly and are moved farther and more often as summer comes to an end. In autumn the pack starts to travel again within its territory, and the pups must keep up. Most pups are almost adult size by October or November. After two or more years in the pack, many leave to search for a mate, establish a new territory, and possibly even start their own pack. Those who stay with the pack may eventually replace a parent to become a breeding animal (alpha). Large packs seem to result from fewer young wolves' leaving the group and from litters' being produced by more than one female. Wolves that leave their packs are known to have traveled as far as 886 km (550 miles).

Wolves are renowned for their wide-ranging travels, and it is not unusual for them to cover 20 km (12 miles) or more in a day. They move and hunt mostly at night, especially in areas populated by humans and during warm weather. The main prey (predation) are large herbivores such as deer, elk (wapiti), moose, bison, bighorn sheep, caribou, and musk oxen (musk ox), which they chase, seize, and pull to the ground. beavers and hares are eaten when available, and wolves in western Canada even fish for Pacific salmon. A large percentage of the animals that wolves kill are young, old, or in poor condition. After making a kill, the pack gorges (consuming some 3 to 9 kg 【7 to 20 pounds】 per animal) and then lingers, often reducing the carcass to hair and a few bones before moving on to look for another meal.

Biologists still disagree on the effect wolves have on the size of prey populations. Wolves may kill livestock and dogs when they have the opportunity, yet many wolves that live near livestock rarely, if ever, kill them. The number of stock killed in North America is small but increasing as wolves expand their range. During the 1990s average annual losses to wolves in Minnesota were 72 cattle, 33 sheep, and 648 turkeys, plus a few individuals of other types of livestock. Stock losses are higher in Eurasia. In some areas wolves survive only by killing livestock and eating livestock carrion and human garbage. Nonetheless, wolves usually avoid contact with humans. There have been few substantiated wolf attacks on humans in North America. Such attacks are unusual but have occurred in Eurasia and India and sometimes have resulted in death.

Wolves have few natural enemies other than man. They can live up to 13 years in the wild, but most die long before that age. Diseases and parasites that can affect wolves include canine parvovirus, distemper (canine distemper), rabies, blastomycosis, Lyme disease, lice (louse), mange, and heartworm. In most areas of the world, humans are the leading cause of death for wolves. In areas of high wolf density and declining prey populations, the major causes of death are killing by other wolves and starvation.

The red wolf (C. rufus) is tawny, reddish, or black. It grows to a length of about 105–125 cm (41–49 inches), excluding the tail, which is 33–43 cm (13–17 inches) long, and weighs about 20–37 kg (44–82 pounds). The red wolf is an endangered species that formerly roamed through the southeastern United States as far west as Texas. Following extinction in the wild, captive-reared red wolves were reintroduced to coastal North Carolina. A small population of fewer than 100 has become established, but the species is threatened by hybridization with coyotes. Some experts believe the red wolf to be a subspecies of gray wolf or a hybrid between the gray wolf and the coyote (sometimes called prairie, brush, or little wolf.)

The critically endangered Abyssinian wolf (C. simensis) also looks similar to the coyote. It lives in a few isolated areas of grassland and heath scrub at high elevations in Ethiopia. Although they live in packs, the wolves hunt alone for rodents and other small mammals.

The Antarctic wolf (Dusicyon australis), now extinct, was actually a South American fox of the Falkland Islands. The dire wolf (C. dirus), was common in western North America during the Pleistocene Epoch but is now extinct. It was the largest known wolf, being half again as large as the modern gray wolf. For other animals that are called wolves but do not belong to the genus Canis, see maned wolf, aardwolf, and Tasmanian wolf (thylacine).