The genealogy of the gods is complicated. There was not one uniform story all the ancient Greeks and Romans believed. One poet could directly contradict another. Parts of stories don't make sense, seemingly happening in reverse order or contradicting something else that was just said.
You shouldn't throw up your hands in despair, though. Familiarity with the genealogy doesn't mean your branches always go in one direction or that your tree looks like the one your neighbor prunes. However, since the ancient Greeks traced their ancestry and that of their heroes to the deities, you should have at least a passing acquaintance with the lineages.
Further back in mythological time than even the gods and goddesses are their ancestors, the primordial powers.
Other pages in this series look at some of the genealogical relationships among the primordial powers and their other descendants (Chaos and Its Descendants, Titans' Descendants, and Descendants of the Sea). This page shows the generations referred to in the mythological genealogies.
Generation 0 - Chaos, Gaia, Eros, and Tartaros
In the beginning were primordial forces. Accounts differ as to how many, but Chaos was probably the first. The Ginnungagap of Norse mythology is similar to Chaos, a sort of nothingness, black hole, or chaotic, swirling disorder or state of conflict. Gaia, the Earth, came next. Eros and Tartaros may also have sprung into existence at about the same time. This is not a numbered generation because these forces were not generated, born, created, or otherwise produced. Either they were always there or they materialized, but the idea of generation involves some sort of creation, so the forces of Chaos, the earth (Gaia), love (Eros), and Tartaros are before the first generation.
The earth (Gaia/Gaea) was the great mother, a creator. Gaia created and then mated with the heavens (Ouranos) and the sea (Pontos). She also produced, but did not mate with the mountains.
From Gaia's union with the heavens (Ouranos/Uranus [Caelus]) came the Hecatonchires (hundred-handers; by name, Kottos, Briareos, and Gyes), the three cyclops/cyclopes (Brontes, Sterope, and Arges), and the Titans
- (Kronos [Cronus],
- Rheia [Rhea],
- Kreios [Crius],
- Koios [Coeus],
- Phoibe [Phoebe],
- Okeanos [Oceanus],
- Theia [Thea],
- Iapetos [Iapetus],
- Mnemosyne, and
Other titans, like Prometheus are also of this generation, and cousins of these early Olympians.
From the mating of Zeus and Hera came
In case it is not completely clear in this table where brothers marry sisters, Kronos (Cronos), Rheia (Rhea), Kreios, Koios, Phoibe (Phoebe), Okeanos (Oceanos), Tethys, Hyperion, Theia, Iapetos, Mnemosyne, and Themis are all offspring of Ouranos and Gaia. Likewise, Zeus, Hera, Poseidon, Hades, Demeter and Hestia are all offspring of Kronos and Rheia.
Sources: Timothy Gantz: Early Greek Myth
Hesiod Theogony, translated by Norman O. Brown
Myth in Daily Life | What Is Myth? | Myths vs. Legends | Gods in the Heroic Age - Bible vs. Biblos | Creation Stories | Uranos' Revenge | Titanomachy | Olympian Gods and Goddesses | Five Ages of Man | Philemon and Baucis | Prometheus | Trojan War | Bulfinch Mythology | Myths and Legends | Golden Fleece and the Tanglewood Tales, by Nathaniel Hawthorne