Werewolves, also known as lycanthropes, are mythological humans with the ability to shapeshift into wolves or wolf-like creatures, either purposely, being bitten by another werewolf or after being placed under a curse. The medieval chronicler Gervase of Tilbury associated the transformation with the appearance of the full moon; however, there is evidence that the association existed among the ancient Greeks, appearing in the writings of Petronius. This concept was rarely associated with the werewolf until the idea was picked up by Gervase. Shape-shifters similar to Werewolves are common in tales from all over the world, though most of them involve animal forms other than wolves.

Werewolves are a frequent subject of modern fictional books and films, although fictional Werewolves have been attributed traits distinct from those of original folklore, most notably the vulnerability to silver bullets.

Many authors have speculated that werewolf and vampire legends may have been used to explain serial killings in less rational ages. This theory is given credence by the tendency of some modern serial killers to indulge in practices commonly associated with Werewolves, such as cannibalism, mutilation, and cyclic attacks. The idea (although not the terminology) is well explored in Sabine Baring-Gould's seminal work The Book of Werewolves.

Until the 20th century, wolf attacks on humans were an occasional, but widespread feature of life in Europe. Some scholars have suggested that it was inevitable that wolves, being the most feared predators in Europe, were projected into the folklore of evil shapeshifters. This is said to be corroborated by the fact that areas devoid of wolves typically use different kinds of predator to fill the niche; werehyenas in Africa, weretigers in India, as well as werepumas ("runa uturunco") and werejaguars ("yaguaraté-abá" or "tigre-capiango") of southern South America.

Writer Ian Woodward theorized in his The werewolf Delusion (1978) that the werewolf legend first developed when the Greeks, Romans, Celts and Germanic tribes were still in good relation with one another;

“When one race began to partition off its particular identity from the other, the superstition became slightly modified- in some cases intensifying its grip on the ancient imagination-according to local needs and cultures. With the growth of culture, too, came the growth of supernaturalism, from the roots of which grew werewolfery.”

—“The werewolf Delusion”, Ian Woodward, Paddington Press, 1978

However, some scholars, both modern and historical trace the origin back to the Paleolithic, specifically from Ircània, a region in ancient Persia, south-east to the Caspian sea.

In his Man into Wolf (1948), anthropologist Robert Eisler drew attention to the fact that many Indo-European tribal names and some modern European surnames mean "wolf" or "wolf-men". This is argued by Eisler to indicate that the European transition from fruit gathering to predatory hunting was a conscious process, simultaneously accompanied by an emotional upheaval still remembered in humanity's subconscious, which in turn became reflected in the later medieval superstition of Werewolves.

Some spiritualist authors have proposed that historical Werewolves, rather than being physical entities, were the astral projections of certain peoples hatred or anger. These authors have argued that the comparatively fewer cases of lycanthropy in modern times has nothing to do with the extermination of wolves, but rather, is a manifestation of modern mans more evolved spiritual state.

Some modern researchers have tried to explain the reports of werewolf behaviour with recognised medical conditions. Dr Lee Illis of Guy's Hospital in London wrote a paper in 1963 entitled On Porphyria and the Aetiology of Werewolves, in which he argues that historical accounts on Werewolves could have in fact been referring to victims of congenital porphyria, stating how the symptoms of photosensitivity, reddish teeth and psychosis could have been grounds for accusing a sufferer of being a werewolf. This is however argued against by Woodward, who points out how mythological Werewolves were almost invariably portrayed as resembling true wolves, and that their human forms were rarely physically conspicuous as porphyria victims. Others have pointed out the possibility of historical Werewolves having been sufferers of hypertrichosis, a hereditary condition manifesting itself in excessive hair growth. However, Woodward dismissed the possibility, as the rarity of the disease ruled it out from happening on a large scale, as werewolf cases were in medieval Europe. People suffering from Downs Syndrome have been suggested by some scholars to have been possible originators of werewolf myths. Rabies has been suggested as being a likely originator of werewolf lore, seeing as how the symptoms of rabies bear some similarities to those manifested by Werewolves. The Roman poet Ovid described the symptoms of Lycaon, one of the first mythological Werewolves;

In vain he attempted to speak; from that very instant

His jaws were bespluttered with foam, and only he thirsted

For blood, as he raged among flocks and panted for slaughter.

His vesture was changed into hair, his limbs became crooked;

A wolf-he retains yet large trace of his ancient expression,

Hoary he is afore, his countenance rabid,

His eyes glitter savagely still, the picture of fury

These symptoms are argued by Woodward as having remarkable similarities to those shown by rabies victims, despite the fact that Ovid was describing what was then considered a werewolf. According to some European traditions, being bitten by a werewolf could result in the victim turning into one. Being bitten by a rabid wolf or person would have spread the condition in the same way. To the medieval mind, a rabid wolf or person would have been seen as a werewolf, especially if a person was attacked by one and subsequently developed rabid symptoms.

Werewolves were said to bear tell-tale traits in European folklore. These included the meeting of both eyebrows at the bridge of the nose, curved fingernails, low set ears and a swinging stride. One method of identifying a werewolf in its human form was to cut the flesh of the accused, under the pretense that fur would be seen within the wound. A Russian superstition recalls a werewolf can be recognised by bristles under the tongue. The appearance of a werewolf in its animal form varies from culture to culture, though they are most commonly portrayed as being indistinguishable from ordinary wolves save for the fact that they have no tail (a trait thought characteristic of witches in animal form), and that they retain human eyes and voice. After returning to their human forms, Werewolves are usually documented as becoming weak, debilitated and undergoing painful nervous depression. Many historical Werewolves were written to have suffered severe melancholia and manic depression, being bitterly conscious of their crimes. One universally reviled trait in medieval Europe was the werewolf's habit of devouring recently buried corpses, a trait which is documented extensively, particularly in the Annales Medico-psychologiques in the 19th century. Fennoscandian Werewolves were usually old women who possessed poison coated claws and had the ability to paralyse cattle and children with their gaze. Serbian vulkodlaks traditionally had the habit of congregating annually in the winter months, where they would strip off their wolf skins and hang them from trees. They would then get a hold of another vulkodlaks skin and burn it, releasing the vulkodlak from whom the skin came from its curse. The Haitian jé-rouges typically try to trick mothers into giving away their children voluntarily by waking them at night and asking their permission to take their child, to which the disoriented mother may either reply yes or no.

Most werewolf legends indicate that the transformation is usually preceded by extreme restlessness and anxiety. As the transformation takes place, the victim is struck by convulsions and contractions before finally retreating to the nearest wood in the form of a lupine animal.

Various methods for becoming a werewolf have been reported, one of the simplest being the removal of clothing and putting on a belt made of wolfskin, probably as a substitute for the assumption of an entire animal skin (which also is frequently described). In other cases, the body is rubbed with a magic salve. To drink water out of the footprint of the animal in question or to drink from certain enchanted streams were also considered effectual modes of accomplishing metamorphosis. The 16th century Swedish writer Olaus Magnus says that the Livonian Werewolves were initiated by draining a cup of specially prepared beer and repeating a set formula. Ralston in his Songs of the Russian People gives the form of incantation still familiar in Russia. According to Russian lore, a child born on December 24 shall be a werewolf. Folklore and literature also depict that a werewolf can be spawned from two werewolf parents.

In Italy, France and Germany, it was said that a man could turn into a werewolf if he, on a certain Wednesday or Friday, slept outside on a summer night with the full moon shining directly on his face.

In Galician, Portuguese, and Brazilian folklore, it is the seventh of the sons (but sometimes the seventh child, a boy, after a line of six daughters) who becomes a werewolf (Lobisomem). In Portugal, the seventh daughter is supposed to become a witch and the seventh son a werewolf; the seventh son often gets the Christian name "Bento" (Portuguese form of "Benedict", meaning "blessed") as this is believed to prevent him from becoming a werewolf later in life. In Brazil, the seventh daughter becomes a headless (replaced with fire) horse called "Mula-sem-cabeça" (Headless Mule). The belief in the curse of the seventh son was so widespread in Northern Argentina (where the werewolf is called the lobizón), that seventh sons were frequently abandoned, ceded in adoption, or killed. A 1920 law decreed that the President of Argentina is the official godfather of every seventh son. Thus, the State gives a seventh son one gold medal in his baptism and a scholarship until his twenty first year. This effectively ended the abandonments, but there still persists a tradition in which the President godfathers seventh sons.

Werewolves have several described weaknesses, the most common being an aversion to wolfsbane (a plant that supposedly sprouted from weeds watered by the drool of Cerberus while he was brought out of Hades by Heracles). Unlike vampires, Werewolves are not harmed by religious artifacts such as crucifixes and holy water.

Another vulnerability is to use a weapon of silver (bullet, knife etc). To stab a werewolf with a silver dagger, or to shoot it with a silver bullet is said to not only kill a werewolf, but to also cause it agony in the time before it dies, rather resembling being slowly burned from the inside. This particular vulnerability is a modern addition to the legends and does not appear before the 19th century.

In many countries, rye and mistletoe were considered effective safeguards against werewolf attacks. Mountain ash is also considered effective, with one Belgian superstition stating that no house was safe unless under the shade of a mountain ash.

The first feature film to use an anthropomorphic werewolf was werewolf of London in 1935 establishing the canon that the werewolf always kills whom he loves most. The main werewolf of this film is a dapper London scientist who retains some of his style and most of his human features after his transformation, as lead actor Henry Hull was unwilling to spend long hours being made up by makeup artist Jack Pierce. Universal Studios drew on a Balkan tale of a plant associated with lycanthropy as there was no literary work to draw upon, unlike the case with vampires. There is no reference to silver nor other aspects of werewolf lore such as cannibalism.

However, he lacks warmth, and it is left to the tragic character Talbot played by Lon Chaney Jr. in 1941's The Wolf Man to capture the public imagination. With Pierce's makeup more elaborate this time, this catapulted the werewolf into public consciousness. Sympathetic portrayals are few but notable; the comedic but tortured protagonist David Naughton in An American werewolf In London, and a less anguished and more confident and charismatic Jack Nicholson in the 1994 film Wolf. Other Werewolves are decidedly more willful and malevolent, such as those in the novel The Howling and its subsequent sequels and film adaptations.

The form a werewolf assumes was generally anthropomorphic in early films such as The Wolf Man and werewolf of London, but larger and powerful wolf in many later films.

The process transmogrification is often portrayed as painful in film and literature within the horror genre. The resulting wolf is typically cunning but merciless and prone to killing and eating people without compunction, regardless of the moral character of its human counterpart.

Werewolves are often depicted as immune to damage caused by ordinary weapons, being vulnerable only to silver objects, such as a silver-tipped cane, bullet or blade; this attribute was first adopted cinematically in The Wolf Man. This negative reaction to silver is sometimes so strong that the mere touch of the metal on a werewolf's skin will cause burns. Current-day werewolf fiction almost exclusively involves lycanthropy being either a hereditary condition or being transmitted like an infectious disease by the bite of another werewolf. In some fiction, the power of the werewolf extends to human form, such as invulnerability, super-human speed and strength and falling on their feet from high falls. Also aggressiveness and animalistic urges may be harder to control (hunger, sexual harassment ). Usually in these cases the abilities are diminished in human form. In other fictions it can even be cured by medicine men or even antidotes.

Fantastic literature sometimes includes the painful element to the change, but often does not. For example, J.K. Rowling maintains the painful transition between forms while Charles de Lint, Terry Pratchett, Fritz Leiber, and myriad others reach back to the non-painful medieval literary sources.

The 1961 Hammer film The Curse of the werewolf, adapted from the 1933 novel The werewolf of Paris by American author Guy Endore, in 1961 draws on traditional legends of a child born on christmas eve being cursed.