The idea persisted through the Middle Ages, where the agency causing shapeshifting is usually a sorcerer or witch, and into the modern period. It remains a common trope in modern fantasy, children's literature and works of popular culture. The most common form of shapeshifting myths is that of therianthropy, which is the transformation of a human being into an animal (i.e. werewolves or vampires) or conversely, of an animal into human form.