The Theogony (Greek: Θεογονία, Theogonía), a poem by Hesiod, is the earliest and most important systematic account of Greek mythology. Little is known of Hesiod other than that he was a farmer in Boeotia, probably in the 7th century BCE. This background is reflected in the opening of the Theogony, in which Hesiod is inspired by the muses while pasturing sheep on Mount Helicon. The poem goes on to describe the creation of the world out of primeval chaos, the genealogy of the Gods, the struggle of the Titans and Olympians, and the divine parentage of human heroes.
Hesiod wrote in a similar epic dialect to Homer, though he is often seen as reflecting a later development. The relatively systematic worldview presented in the Theogony has made it a key point of comparison with the earliest Greek philosophers, a number of whom continued to use poetic forms and indeed myth.
Free online texts
Gutenberg: Hesiod, the Homeric Hymns, and Homerica.
Sacred-Texts: Theogony, translated by H.G. Evelyn-White. HTML format.
Theoi: Theogony ,translated by H.G. Evelyn-White. HTML format.
University of Adelaide (Internet Archive): Hesiod, the Homeric Hymns, and Homerica, translated by H.G. Evelyn-White. EPUB, HTML and MOBi formats.
Wikisource: The Theogony, translated by H.G. Evelyn-White (1920). Multiple formats.
Wikisource: ΘΕΟΓΟΝΙΑ. Greek text. Multiple formats.
History of Ancient Greece: The Greek Genesis, podcast by Ryan Stitt.
History of Ancient Greece: Oligarchs and Hesiod, podcast by Ryan Stitt.
Librivox: Works and Days, The Theogony, and The Shield of Heracles - Public domain audiobook.
Literature and History: Before Orthodoxy - Hesiod's Theogony - Ancient Greece's Creation Story. Podcast and transcript.
The Great Conversation: Further reading at Tom's Learning Notes
Hesiod: Works and Days.
Ancient Greek resources: Learn to read Greek classics in the original.
Bloom's Western Canon: The Theogony is listed.