The author attempts a definitive study of the nightmare, introducing evidence from psychiatry, history, literature, and mythology. As a result of these investigations, he has evolved a theory that concerns the relationship between religion and the burden of guilt that everyone supposedly inherits from a primordial conflict arising from repressed incestuous wishes. Phenomena of the nightmare are considered to be due to the invocation, by all peoples and all faiths through the ages, of mythological creatures, such as werewolves, vampires, incubi and succubi, devils, and witches, as projections of guilt. The author cites two essential attributes of the nightmare: 1. It arises from within. 2. It is of mental origin, a sexual wish in a state of repression. In this respect, the nightmare supposedly differs from ordinary erotic dreams only in that the erotic wish has been repressed. He concludes that if the wish were not repressed, fear would not