An American author, principally known for his award-winning science fiction and fantasy novels and short stories.

He is best known for his sequences of novels, especially the World of Tiers (1965–93) and Riverworld (1971–83) series. He is noted for the pioneering use of sexual and religious themes in his work, his fascination for, and reworking of, the lore of celebrated pulp heroes, and occasional tongue-in-cheek pseudonymous works written as if by fictional characters.