A werewolf (also lycanthrope or wolfman) in folklore is a person who shapeshifts into a wolf or wolflike creature, either purposely, by using magic, 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 during among the Ancient Greeks, appearing in Petronius's writings. This concept was rarely associated with the werewolf until the idea was picked up by fiction writers. In popular culture, a werewolf can be killed only if shot by a silver bullet, although this was not a feature of the folk legends.

The process of transmogrification is often portrayed as painful in film and literature. 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. The form a werewolf assumes is not always that of an ordinary wolf, but often anthropomorphic or otherwise larger and more powerful than an ordinary wolf. Many modern werewolves are supposedly immune to damage caused by ordinary weapons, being vulnerable only to silver objects (usually a bullet or blade). 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.

More recently, the portrayal of werewolves has taken an even more sympathetic turn in some circles. With the rise of environmentalism and other back-to-nature ideals, the werewolf has come to be seen by some authors as a representation of humanity allied more closely with nature. Some recent fiction also discards the idea that the werewolf dominates the mind when one transforms, and instead postulates that the wolf form can be used at will, with the lycanthrope retaining its human thought processes and intelligence.