In ancient Greece, “no other practice was so universal than the consultation of oracles.” These holy sites were found in 260 locations around the Greek-speaking world, and they were considered the “most satisfactory means of ascertaining the future.” These sanctuaries were “set apart from the profane, ordinary world”, and were restricted to natural locations where the divine was thought to be “especially present.” They were set aside for special spiritual functions, and a state of purity was generally required of its participants. A historical tendency toward providing haven for criminals was based on the fear of shedding blood in a holy place, and the “fear that the evil magic would emanate from his curse.”

Of course, the Oracle of Delphi was one of the greatest religious institutions in ancient Greece, and one which played a significant role not only in the formation and collective decisions of Hellenic localities and city-states, but also in the personal lives of Greeks. The site was dedicated to the god Apollo, and the Greeks believed the god spoke his oracles through his prophetess, known as the Pythia. The judgments and decisions rendered by the oracle were so important to the Greeks, that they often put them above all other interests, even security threats posed by the likes of the Persians, and Delphi was popular even amongst outsiders.

Aside from Delphi, the most prominent oracles were located near Colophon, Xanthus, Claros, Branchidae, Olympia, and Dodona, and all the Greek oracles have been found in places with “extraordinary natural features” connected to their powers. The Oracle of Dodona was the oldest of all, and it was one of the few dedicated to Zeus and his consort goddess Dione. The oracular utterances heard at the oracles are generally referred to as “smokiness” in Greek, but at Dodona, the answers came in natural sounds and other peripheral phenomena.

Unlike Apollo, Zeus traditionally had only one oracle: the Oracle of Dodona. The Oracle of Dodona was considered to be the oldest oracle in Greece, and was originally dedicated to Rhea, Zeus' mother (this was widely known in Greece), but he displaced her over time. The Oracle (always a woman) would listen to the sound of the wind in the leaves of oak trees dedicated to Zeus and interpret the sound as the god's voice. Much later, Zeus also "adopted" the Oracle at Siwa in the Libyan desert. The Siwa Oracle was one of the "Sibyls" of the ancient world, and when she was visited by Alexander the Great, she predicted his rise to power and informed him of his divine heritage as Zeus's son.

Despite the interest in Greek oracles, modern knowledge remains limited in certain respects, because cult history has become so intertwined with mythology that the lines separating one from another have been nearly lost. Modern scholars studying the oracles have tried to pull the shroud of mythology away from historical facts to illuminate the realities of the Greek cults, but the job has often proved trickier than imagined. If anything, the work of scholars has deepened the mysticism of the oracles rather than dispel it, in large measure due to documenting fascinating and mysterious stories about them.

For example, much has been written about natural vapors and fumes that may have caused psychoactive effects in petitioners at oracles. Modern civilizations at times employ this argument to excuse the supposed ignorance of the ancient Greeks, and in some cases, such vapors have been identified and may indeed explain the activities at certain oracles. However, archaeologist Jelle de Boer, intimately associated with some of the most iconic Greek oracles, cautioned that “something can be extra-natural without being outright supernatural."