Monsters, werewolves, witches, and fairies remain a strong presence in our stories and dreams. Their roots predate Christianity and are widespread over Western Europe. The author’s extensive analysis of Western European legends reveals an almost forgotten religious concept: that every individual owns three souls and that one, the Double, can leave the physical body to journey where it chooses. Claude Lecouteux reveals the origin and significance of this belief, and follows its transforming features through the ages.