The grey wolf or gray wolf, often known simply as wolf, is the largest wild member of the Canidae family.

European wolves may weigh 38.5 kilograms (85 lb), in general, height varies from 0.6 to 0.95 meters (24 to 37 in) at the shoulder. Grey wolves are sexually dimorphic, with females in any given wolf population typically weighing 20% less than males. Females also have narrower muzzles and foreheads; slightly shorter, smoother furred legs; and less massive shoulders. Gray wolves can measure anywhere from 1.3 to 2 meters (4.3 to 6.6 ft) from nose to the tip of the tail, which itself accounts for approximately one quarter of overall body length.

Gray wolves rely on their stamina rather than speed for hunting. Their narrow chests and powerful backs and legs facilitate efficient locomotion. They are capable of covering several miles trotting at about 10 kilometers per hour (6 mph), and have been known to reach speeds approaching 65 kilometers per hour (40 mph) during a chase.

Gray wolf paws are able to tread easily on a wide variety of terrains, especially snow. There is a slight webbing between each toe, which allows them to move over snow more easily than comparatively hampered prey. Gray wolves are digitigrade, which, with the relative largeness of their feet, helps them to distribute their weight well on snowy surfaces. The front paws are larger than the hind paws, and have a fifth digit, the dewclaw, that is absent on hind paws.Bristled hairs and blunt claws enhance grip on slippery surfaces, and special blood vessels keep paw pads from freezing. Scent glands located between a wolf's toes leave trace chemical markers behind, helping the wolf to effectively navigate over large expanses while concurrently keeping others informed of its whereabouts. Unlike dogs and western coyotes, gray wolves have a lower density of sweat glands on their paws.

Wolves have bulky coats consisting of two layers. The first layer is made up of tough guard hairs that repel water and dirt. The second is a dense, water-resistant undercoat that insulates. The undercoat is shed in the form of large tufts of fur in late spring or early summer (with yearly variations). A wolf will often rub against objects such as rocks and branches to encourage the loose fur to fall out. The undercoat is usually gray regardless of the outer coat's appearance. Wolves have distinct winter and summer pelages that alternate in spring and autumn. Females tend to keep their winter coats further into the spring than males.

Fur coloration varies greatly, running from gray to gray-brown, all the way through the canine spectrum of white, red, brown, and black. These colors tend to mix in many populations to form predominantly blended individuals, though it is not uncommon for an individual or an entire population to be entirely one color (usually all black or all white).

Wolves and most larger dogs share identical dentition. The maxilla has six incisors, two canines, eight premolars, and four molars. The mandible has six incisors, two canines, eight premolars, and six molars. The fourth upper premolars and first lower molars constitute the carnassial teeth, which are essential tools for shearing flesh. The long canine teeth are also important, in that they hold and subdue the prey. Capable of delivering up to 10,000 kilopascals (1,500 psi) of pressure, a wolf's teeth are its main weapons as well as its primary tools. This is roughly twice the pressure that a domestic dog of similar size can deliver. The dentition of grey wolves is better suited to bone crushing than those of other modern canids.

Generally, mating occurs between January and April—the higher the latitude, the later it occurs. A pack usually produces a single litter unless the breeding male mates with one or more subordinate females. During the mating season, breeding wolves become very affectionate with one another in anticipation of the female's ovulation cycle. The pack tension rises as each mature wolf feels urged to mate. During this time, the breeding pair may be forced to prevent other wolves from mating with one another. When the breeding female goes into estrus (which occurs once per year and lasts 5–14 days), she and her mate will spend an extended time in seclusion.

The gestation period lasts between 60 and 63 days. The pups, which weigh about 0.5-kilogram (1 lb) at birth, are born blind, deaf, and completely dependent on their mother. The average litter size is 5–6 pups. The pups reside in the den and stay there for two months. The den is usually on high ground near an open water source, and has an open chamber at the end of an underground or hillside tunnel that can be up to a few meters long. During this time, the pups will become more independent, and will eventually begin to explore the area immediately outside the den before gradually roaming up to a mile away from it at around five weeks of age. They begin eating regurgitated foods after two weeks of feeding on milk, which in wolves has less fat and more protein and arginine than dog milk. By this time, their milk teeth have emerged—and are fully weaned by 10 weeks. During the first weeks of development, the mother usually stays with her litter alone, but eventually most members of the pack will contribute to the rearing of the pups in some way. After two months, the restless pups will be moved to a rendezvous site, where they can stay safely while most of the adults go out to hunt. One or two adults stay behind to ensure the safety of the pups. After a few more weeks, the pups are permitted to join the adults if they are able, and will receive priority on anything killed, their low ranks notwithstanding. Letting the pups fight for eating privileges results in a secondary ranking being formed among them, and allows them to practice the dominance/submission rituals that will be essential to their future survival in pack life. During hunts, the pups remain ardent observers until they reach about eight months of age, by which time they are large enough to participate actively.

At birth, wolf pups tend to have darker fur and blue irises that will change to a yellow-gold or orange color when the pups are between 8 and 16 weeks old.

Wolves typically reach sexual maturity after two or three years, at which point many of them will be compelled to leave their birth packs and seek out mates and territories of their own. Wolves that reach maturity generally live six to ten years in the wild.. Pups die when food is scarce.