The most widespread is the gray wolf, C. lupus, of circumpolar distribution; in addition to the domestic dog, its subspecies include the timber wolf, the arctic wolf, and the dingo. Extinct in W Europe except in a few isolated pockets, it is still found in SE Europe, Russia, and much of Asia. In the New World it is found in wilderness forests and tundra from Greenland and the shores and islands of the Arctic Ocean to the extreme N United States. There is a healthy population in Alaska, but it has had endangered-species status in the 48 contiguous United States (except for Minnesota, where it has been listed as threatened). Thus protected, it has steadily increased its range since the late 1980s, especially in the Great Lakes region in Minnesota, Wisconsin, and Michigan and in the states surrounding Yellowstone National Park, where Canadian wolves were introduced in 1995 in the hope of restoring balance to the Yellowstone ecosystem. Canadian wolves were also introduced in central Idaho in 1995 and 1996, and natural reproduction has since steadily increased the numbers of both populations. Wolves have also migrated into NW Montana from Canada and established themselves there.

The gray wolf is similar in appearance to a German shepherd, with a thick, shaggy coat, erect ears, and a bushy tail. Its fur is usually gray mixed with black and brown but may be nearly black or, in the Arctic, nearly white. An average-sized adult male is about 3 ft (90 cm) high at the shoulder and 4 ft (120 cm) long, excluding the tail, and weighs about 100 lb (45 kg); some individuals weigh twice as much.

Active mostly at night, gray wolves prey on birds and small mammals and on weak members of larger species, such as deer; they also eat vegetable matter and some carrion. They can run at speeds of up to 35 mi (56 km) per hour and can clear 16 ft (4.9 m) in a single bound. While hunting they can maintain a speed of about 20 mi (32 km) per hr for many hours, eventually wearing down even the swiftest prey. They roam over large areas and may migrate in response to migrations by or numerical fluctuations in their prey species.

Gray wolves hunt singly and in family groups, called packs, which typically include about five individuals. Under severe conditions, especially in winter, several families may join together, forming a pack of up to 30 individuals, rarely more. During the mating season a wolf pair establish a den, usually in a cave or underground burrow, in which they raise the young; both parents bring home food. A pair is believed to remain mated for life.

Because of farmers' fears of raids on livestock, which wolves usually take only when wild prey is unavailable, gray wolves have been hunted ruthlessly, resulting in their extermination in all but the most sparsely populated areas. North American gray wolves have not been known to attack humans without provocation, although Siberian gray wolves have on occasion attacked riders of horses or horse-drawn vehicles. There are many stories of human children being raised by gray wolves, particularly in India, but none has been authenticated.

The red wolf, C. rufus, is a smaller species that varies in color from reddish gray to nearly black. It has been nearly eradicated from most of its range in the forest and brush country of the S central United States and is listed as endangered. However, captive breeding programs are slowly increasing its numbers, and some have been reintroduced to the wild. The red wolf is similar in behavior to, and may be a hybrid of, the prairie wolf, C. latrans, better known as the coyote. Smallest of the wolves, coyotes are still widespread in W North America. Real estate development in their traditional habitat, combined with the opening up of the ecological niche formerly filled by gray wolves and mountain lions, has prompted coyotes to greatly increase their range; they are now common in the Northeast and have developed small populations in large urban centers such as Chicago and New York City.

Wolves are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Carnivora, family Canidae.