Ancient Greek festivals were major religious events that recurred annually, every two years, or every four years.

Many, perhaps most, ancient Greeks and Romans did not belong to any particular sect. The average person in the ancient world would likely worship the local city god, participate in the Roman imperial cult, honor the patron god of a profession, and pay special devotion to whichever deity seems most appealing or likely to help with a particular concern. But there were also a number of special religious or philosophical groups in the ancient world to which one could belong. For example, an upper-class person might be drawn to a philosophy like Stoicism or Epicureanism, a Roman soldier might be initiated into Mithraism, and a person of any rank might be a special devotee of Dionysus. These specific cults, sects and schools are explored in this section.

. The cult practices of the Hellenes extended beyond mainland Greece to the islands and coasts of Ionia in Asia Minor, to Magna Graecia (Sicily and southern Italy) and to scattered Greek colonies in the Western Mediterranean, such as Massilia (Marseille). Greek examples tempered Etruscan cult and belief to inform much of Roman religion.

Different cities worshipped different deities: Athens had Athena; Sparta had Artemis; Corinth was a center for the worship of Aphrodite; Delphi and Delos had Apollo; Olympia had Zeus, and so on down to the smaller cities and towns.

"perirrhanteria", which was a marble water basin set up near altars