The Olympians were a race of deities, primarily consisting of a third and fourth generation of immortal beings, worshipped as the principal gods of the Greekpantheon and so named because of their residency atop Mount Olympus. They gained their supremacy in a ten-year-long war of gods, in which Zeus led his siblings to victory over the previous generation of ruling gods, the Titans. They were a family of gods, the most important consisting of the first generation of Olympians, offspring of the Titans Cronus and Rhea: Zeus, Poseidon, Hera, Demeter and Hestia, along with the principal offspring of Zeus: Athena, Apollo, Artemis, Ares, Aphrodite, Hephaestus, Hermes, and Dionysus. Although Hades was a major deity in the Greek pantheon, and was the brother of Zeus and the other first generation of Olympians, his realm was far away from Olympus in the underworld, and thus he was not usually considered to be one of the Olympians. Olympic gods can be contrasted to chthonic gods including Hades, by mode of sacrifice, the latter receiving sacrifices in a bothros or megaron rather than at an altar. The canonical number of Olympian gods was twelve, but besides the principal Olympians listed above, there were many other residents of Olympus, who thus might be considered to be Olympians. Heracles became a resident of Olympus after his apotheosis and married another Olympian resident Hebe. According to Hesiod, the children of Styx: Zelus, Nike, Cratos, and Bia, "have no house apart from Zeus, nor any dwelling nor path except that wherein God leads them, but they dwell always with Zeus." Some others who might be considered Olympians, include the Muses, the Graces, Iris, Dione, Eileithyia, the Horae, and Ganymede.
Besides the twelve Olympians, there were many other various cultic groupings of twelve gods throughout ancient Greece. The earliest evidence of Greek religious practice involving twelve gods comes no earlier than the late sixth century BC. According to Thucydides, an altar of the twelve gods was established in the agora of Athens by the archonPisistratus, in c. 522 BC. The altar became the central point from which distances from Athens were measured and a place of supplication and refuge. Olympia apparently also had an early tradition of twelve gods. The Homeric Hymn to Hermes has the god Hermes divide a sacrifice of two cows he has stolen from Apollo, into twelve parts, on the banks of the riverAlpheius : Pindar, in an ode written to be sung at Olympia c. 480 BC, has Heracles sacrificing, alongside the Alpheius, to the "twelve ruling gods": Another of Pindar's Olympian odes mentions "six double altars". Herodorus of Heraclea also has Heracles founding a shrine at Olympia, with six pairs of gods, each pair sharing a single altar. Many other places had cults of the twelve gods, including Delos, Chalcedon, Magnesia on the Maeander, and Leontinoi in Sicily. As with the twelve Olympians, although the number of gods was fixed at twelve, the membership varied. While the majority of the gods included as members of these other cults of twelve gods were Olympians, non-Olympians were also sometimes included. For example, Herodorus of Heraclea identified the six pairs of gods at Olympia as: Zeus and Poseidon, Hera and Athena, Hermes and Apollo, the Graces and Dionysus, Artemis and Alpheus, and Cronus and Rhea. Thus while this list includes the eight Olympians: Zeus, Poseidon, Hera, Athena, Hermes, Apollo, Artemis, and Dionysus, it also contains three clear non-Olympians: the Titan parents of the first generation of Olympians, Cronus and Rhea, and the river god Alpheius, with the status of the Graces being unclear. Plato connected "twelve gods" with the twelve months, and implies that he considered Pluto one of the twelve in proposing that the final month be devoted to him and the spirits of the dead. The Roman poet Ennius gives the Roman equivalents as six male-female complements, preserving the place of Vesta, who played a crucial role in Roman religion as a state goddess maintained by the Vestals.
There is no single canonical list of the twelve Olympian gods. The thirteen gods and goddesses most commonly considered to be one of the twelve Olympians are listed below.