Before we get to the life and rituals of the seers at the oracles, let us set the stage by taking a look at divination and the oracles in ancient Greece.

Divination can be defined as the different techniques used to gain access to knowledge that is not available to normal human perception. It played an important part in the life of the ancient Greeks both in their private lives and in military and political matters.

The Greek word for divination is mantiké, and the Greeks used many different methods. Divination was not only practiced by the oracles but were a part of everyday life. Different types of signs could be interpreted by a seer (mantis). Any unexpected event, e.g. a sneeze, could be a sign. Weather phenomena and other celestial phenomena such as lightning and comets could be signs. An important form of divination was observation of birds - raptors were especially important in this context. When sacrificing, any event could be a sign, but the entrails of the sacrificial animal were especially important for divination: The liver of the animal was studied, and its appearance and form was interpreted. Dreams could also be omens, and in Hellenistic times, astrology became a very important form of divination.
Chresmologoi (meaning reciters/tellers of oracles) were professionals who collected old oracular prophecies. They would recite and interpret the old oracle answers, but they could not give new answers themselves. The Greeks distinguished between seers and chresmologoi.

The use of oracles is probably the kind of divination that is best known today, especially the oracle in Delphi. The use of Oracles was widespread, and more than 60 are known in Greek-speaking areas. Most of them had only local or regional significance, but some were influential in larger parts of the Greek-speaking world. The most important and most well-known was the oracle in Delphi, but some other important oracles were the oracles in Dodona in Epirus, Didyma in Asia Minor and Cumae in Magna Grecia. A variety of techniques were used by the oracles: in some places, the oracle responses were given under divine inspiration, e.g. In Delphi, where the god Apollon gave answers through a priestess known as Pythia. Other techniques used were different types of sortition (drawing lots), interpreting signs e.g. in the entrails of sacrificial animals, or incubation (incubation oracles were predominantly the domain of the healer gods and heroes. After preparatory sacrifices and ritual cleansing, the questioner/patient slept in the inaugurated area. The god/hero sent an oracle response in a dream, and priests would interpret it and prescribe a cure based on it. However, some other oracles also used this technique). One of the more spectacular forms of oracles were the oracles of the dead and oracles of the underworld. The travel author Pausanias describes his own visit to the oracle of the hero Trophonios in the work Description of Greece. It was located next to the town of Lebadeia in Boeotia in central Greece. It was an awe-inspiring experience where, after thorough preparations, he was brought down into the underworld in the form of an underground sanctuary, and the oracle response was revealed to him.

Apollo was the primary oracular god of the Greeks. The second most important was Zeus, and other gods and heroes had their own oracles.

Oracles were in operation for more than a thousand years. It is uncertain when the Greek oracles as we know them began, but they are known from the 8th century BCE and continued to be used until an edict of Theodosius, Valentinian II and Arcadius closed all oracular temples and forbade divination in 391 CE. Not all the oracles we know today were in operation throughout the period and many of the oracles, including Delphi, were in decline for some time before the end.