Greek mythology comprises the collected narratives of Greek gods, goddesses, heroes, and heroines, originally created and spread within an oral-poetic tradition. Our surviving sources of mythology are literary reworkings of this oral tradition, supplemented by interpretations of iconic imagery, sometimes modern ones, sometimes ancient ones, as myth was a means for later Greeks themselves to throw light on cult practices and traditions that were no longer explicable. The historian must sometimes deduce from hints in imagery, such as in vase paintings, and offhand references the recognition of mythic themes tacitly expressed in cult practice.
The general issues in studying myths are discussed in the mythography article.
In their various legends, stories and hymns the gods of ancient Greece are all described as human in appearance: the few chimerical beings such as the Sphinx all have Near Eastern or Anatolian origins. The Greek gods may have birth myths but they are unaging. The gods are nearly immune to wounds and to all sickness, capable of becoming invisible, able to travel vast distances almost instantly, and able to speak through human beings with or without their knowledge. Each has his or her own specific appearance, genealogy, interests, personality, and area of expertise; however, these descriptions arise from a multiplicity of archaic local variants, which do not always agree with one another. When these gods were called upon in poetry, prayer or cult, they are referred to by a combination of their name and epithets, that identify them by these distinctions from other manifestations of themselves. A Greek deity's epithet may reflect a particular aspect of that god's role, as Apollo Musagetes is "Apollo, [as] leader of the Muses." Alternatively the epithet may identify a particular and localized aspect of the god, sometimes thought to be already ancient during the classical epoch of Greece.
In such mythic narratives, these beings are described as a large, multi-generational family. Their oldest members created the world, but succeeding generations overcame the older gods. The Olympian twelve gods most familiar from ancient Greek religion and Greek art are described in epic poems as having appeared in person to the Greeks during the "age of heroes." They provided the struggling ancestors of the Greeks with a limited number of miracles, taught them a selection of useful skills, taught them the methods of worshipping the gods, rewarded virtue and punished vice, and fathered children by humans.