In ancient Greece, deities were regarded as anthropomorphic, immortal, and powerful. They were conceived of as persons rather than abstract concepts, and were described as similar to humans in appearance, albeit larger and more beautiful. The emotions and actions of deities were largely the same as those of humans; they frequently engaged in sexual activity, and were jealous and amoral. Deities were considered far more knowledgeable than humans, and it was believed that they conversed in a language of their own. Their immortality, the defining marker of their godhood, meant that they ceased aging after growing to a certain point. In place of blood, their veins flowed with ichor, a substance which was a product of their diet, and conferred upon them their immortality. Divine power allowed the gods to intervene in mortal affairs in various ways: they could cause natural events such as rain, wind, the growing of crops, or epidemics, and were able to dictate the outcomes of complex human events, such as battles or political situations.
As ancient Greek religion was polytheistic, a multiplicity of gods were venerated by the same groups and individuals. The identity of a deity was demarcated primarily by their name, which could be accompanied by an epithet (a title or surname). Religious epithets could refer to specific functions of a god, to connections with other deities, or to a divinity's local forms. The Greeks honoured the gods by means of worship, as they believed deities were capable of bringing to their lives positive outcomes outside their own control. Greek cult, or religious practice, consisted of activities such as sacrifices, prayers, libations, festivals, and the building of temples. By the 8th century BC, most deities were honoured in sanctuaries (), sacred areas which could include altars and temples, and were typically dedicated to a single deity. Aspects of a god's cult such as the kinds of sacrifices made to them and the placement of their sanctuaries contributed to the distinct conception worshippers had of them.
In addition to a god's name and cult, their character was determined by their mythology (the collection of stories told about them) and their iconography (how they were depicted in ancient Greek art). A deity's mythology told of their deeds, which played a role in establishing their functions, and genealogically linked them to gods with similar functions. The most important works of mythology were the Homeric epics, including the Iliad (750–700 BC), an account of a period of the Trojan War, and Hesiod's Theogony (c. 700 BC), which presented a genealogy of the pantheon. Myths known throughout Greece had different regional versions, which sometimes presented a distinct view of a god according to local concerns. Some myths attempted to explain the origins of certain cult practices, and some may have arisen from rituals. Artistic representations allow us to understand how deities were depicted over time, and works such as vase paintings sometimes substantially predate literary sources. Art contributed to how the Greeks conceived of the gods, and depictions would often assign deities certain symbols, such as the thunderbolt of Zeus or the trident of Poseidon.
The principal figures of the pantheon were the twelve Olympians, thought to live on Mount Olympus and to be connected as part of a family. Zeus was considered the chief god of the pantheon, though Athena and Apollo were honoured in a greater number of sanctuaries in major cities, and Dionysus is the deity who has received the most attention in modern scholarship. Beyond the central divinities of the pantheon, the Greek gods were numerous. Some parts of the natural world, such as the earth, sea, or Sun, were held as divine throughout Greece, while other natural deities, such as the various nymphs and river gods, were primarily of local significance. Personifications of abstract concepts appeared frequently in Greek art and poetry, though many were also venerated in cult, some as early as the 6th century BC. Groups or societies of deities could be purely mythological in importance, such as the Titans, or they could be the subject of substantial worship, such as the Muses or Charites.
Major deities in Greek religion The following section is based upon Walter Burkert's Greek Religion (1985), particularly his chapter "III: The Gods".
Twelve Olympians The main deities of the Greek pantheon were the twelve Olympians. They were believed to reside on Mount Olympus, from which they derived their name, and were connected as part of a familial group which had Zeus at its head. This family included two generations: the first consisted of children of Cronus and Rhea – Zeus, Poseidon, Hera, Demeter, and Hestia – and the second consisted of children of Zeus – Athena, Apollo, Artemis, Ares, Hephaestus, Aphrodite, Hermes, and Dionysus (though Aphrodite and Hephaestus were sometimes said to not be children of Zeus). In myth, the Olympians were preceded by another group of gods, the Titans (among them Cronus and Rhea), who were supplanted by Zeus and the Olympian gods in a war known as the Titanomachy, after which Zeus became ruler of the gods.
In cult, the notion of the twelve gods (or ) is first attested in the latter half of the 6th century BC, when the Altar of the Twelve Gods was constructed in Athens. Around the same time, the Homeric Hymn to Hermes referred to the division of a sacrifice into twelve pieces, and in 484 BC the poet Pindar mentioned the honouring of twelve gods at Olympia. By the classical period (5th–4th centuries BC), this idea of twelve gods had become established. Although the Olympians were consistently considered twelve in number, the individual gods which comprised this group of twelve could differ by region; in particular, Hestia and Dionysus were sometimes excluded. Although Hades is the brother of the first-generation Olympians, he was not included among the twelve Olympians owing to his residency in the underworld. In addition to the canonical twelve Olympians, there were numerous other gods generally believed to live on Olympus.