Oracles, dreams, and omens appear throughout the Histories, as signs or warnings of important future events. Supernatural in origin, and often obscure or ambiguous in meaning, they are manifestations of the divine in the human world and convey an authoritative truth that can only be imperfectly grasped by mortals. While Herodotus usually displays a noncommittal or relativist attitude towards religious matters, he generally accepts the truthfulness of oracular prophecies and often directly quotes oracles in his narrative. Frequently metaphorical or symbolic, the obscurity of oracles and dreams indicates the restricted nature of human knowledge, which can only grasp the future partially and indirectly. The dichotomy between the limitations of human knowledge and the inscrutable workings of fate, which governs human fortune, is a major theme of the Histories. The use of oracles, dreams, and omens as a narrative motif enables Herodotus to lend a religious dimension to the complex network of human actions and motives and construct a moral framework for his main themes that is grounded in a dimly perceived but inescapable divine order underlying the world.

The most important Greek oracle (the term is used for the prophetic institution as well as the message) was that of Apollo at Delphi.