There is evidence of widespread belief in werewolves in medieval Europe. This evidence spans much of the Continent, as well as the British Isles. Werewolves were mentioned in Medieval law codes, such as that of King Cnut, whose Ecclesiastical Ordinances inform us that the codes aim to ensure that “…the madly audacious werewolf do not too widely devastate, nor bite too many of the spiritual flock.’ Liutprand of Cremona reports a rumour that Bajan, son of Simeon I of Bulgaria, could use magic to turn himself into a wolf. The works of Augustine of Hippo had a large influence on the development of Western Christianity, and were widely read by churchmen of the medieval period; and these churchmen occasionally discussed werewolves in their works. Famous examples include Gerald of Wales's Werewolves of Ossory, found in his Topographica Hibernica, and in Gervase of Tilbury's Otia Imperiala, both written for royal audiences.

Gervase reveals to the reader that belief in such transformations (he also mentions women turning into cats and into snakes) was widespread across Europe; he uses the phrase "que ita dinoscuntur" when discussing these metamorphoses, which translates to "it is known". Gervase, who was writing in Germany, also tells the reader that the transformation of men into wolves cannot be easily dismissed, for "...in England we have often seen men change into wolves" ("Vidimus enim frequenter in Anglia per lunationes homines in lupos mutari…"). Further evidence of the widespread belief in werewolves and other human-animal transformations can be seen in theological attacks made against such beliefs; Conrad of Hirsau, writing in the 11th century, forbids the reading of stories in which a person's reason is obscured following such a transformation. Conrad specifically refers to the tales of Ovid in his tract. Pseudo-Augustine, writing in the 12th century, follows Augustine of Hippo's argument that no physical transformation can be made by any but God, stating that "...the body corporeally [cannot], be changed into the material limbs of any animal.'

Marie de France's poem Bisclavret (c. 1200) is another example, in which the eponymous nobleman Bisclavret, for reasons not described, had to transform into a wolf every week. When his treacherous wife stole his clothing needed to restore his human form, he escaped the king's wolf hunt by imploring the king for mercy and accompanied the king thereafter. His behaviour at court was gentle, until his wife and her new husband appeared at court, so much so that his hateful attack on the couple was deemed justly motivated, and the truth was revealed. This lai (a type of Breton sung-poem) follows many themes found within other werewolf tales - the removal of clothing and attempting to refrain from the consumption of human flesh can be found in Pliny the Elder, as well as in the second of Gervase of Tilbury's werewolf stories, about a werewolf by the name of Chaucevaire. Marie also reveals to us the existence of werewolf belief in Breton and Norman France, by telling us the Franco-Norman word for werewolf: garwulf, which, she explains, are common in that part of France, where "...many men turned into werewolves". Gervase also supports this terminology when he tells us that the French use the term "gerulfi" to describe what the English call "werewolves". Melion and Biclarel are two anonymous lais that share the theme of a werewolf knight being betrayed by his wife.

The German word werwolf is recorded by Burchard von Worms in the 11th century, and by Bertold of Regensburg in the 13th, but is not recorded in all of medieval German poetry or fiction. While Baring-Gould argues that references to werewolves were also rare in England, presumably because whatever significance the "wolf-men" of Germanic paganism had carried, the associated beliefs and practices had been successfully repressed after Christianization (or if they persisted, they did so outside of the sphere of literacy available to us), we have sources other than those mentioned above. Such examples of werewolves in Ireland and the British Isles can be found in the work of the 9th century Welsh monk Nennius; female werewolves appear in the Irish work Tales of the Elders, from the 12th century; and Welsh werewolves in the 12th-13th century Mabinogion.

In 1539, Martin Luther used the form beerwolf to describe a hypothetical ruler worse than a tyrant who must be resisted.

The Germanic pagan traditions associated with wolf-men persisted longest in the Scandinavian Viking Age. Harald I of Norway is known to have had a body of Úlfhednar (wolf-coated [men]), which are mentioned in the Vatnsdœla saga, Haraldskvæði, and the Völsunga saga, and resemble some werewolf legends. The Úlfhednar were fighters similar to the berserkers, though they dressed in wolf hides rather than those of bears and were reputed to channel the spirits of these animals to enhance effectiveness in battle. These warriors were resistant to pain and killed viciously in battle, much like wild animals. Úlfhednar and berserkers are closely associated with the Norse god Odin.

The Scandinavian traditions of this period may have spread to Kievan Rus', giving rise to the Slavic "werewolf" tales. The 11th-century Belarusian Prince Vseslav of Polotsk was considered to have been a werewolf, capable of moving at superhuman speeds, as recounted in The Tale of Igor's Campaign:

Vseslav the prince judged men; as prince, he ruled towns; but at night he prowled in the guise of a wolf. From Kiev, prowling, he reached, before the cocks crew, Tmutorokan. The path of Great Sun, as a wolf, prowling, he crossed. For him in Polotsk they rang for matins early at St. Sophia the bells; but he heard the ringing in Kiev.

The situation as described during the medieval period gives rise to the dual form of werewolf folklore in Early Modern Europe. On one hand the "Germanic" werewolf, which becomes associated with the witchcraft panic from around 1400, and on the other hand the "Slavic" werewolf or vlkolak, which becomes associated with the concept of the revenant or "vampire". The "eastern" werewolf-vampire is found in the folklore of Central and Eastern Europe, including Hungary, Romania and the Balkans, while the "western" werewolf-sorcerer is found in France, German-speaking Europe and in the Baltic.