In Greek mythology Lētō' (Greek: Λητώ, Lato in Dorian Greek, the "hidden one") is known to be a daughter of the Titans Coeus and Phoebe, and in the Olympian scheme of things, Zeus is the father of her twins, Apollo and Artemis. Still, Leto is scarcely to be conceived apart from being pregnant and finding a suitable place to be delivered of Apollo, the second of her twins. This is her one active mythic role: once Apollo and Artemis are grown, Leto withdraws, to remain a dim, benevolent matronly figure upon Olympus, dark and mild, her part already played.

In Crete, at Dreros, Marinatos uncovered an 8th-century post-Minoan hearth house temple in which there were unique hammered brass figures of Apollo, Artemis and Leto. Walter Burkert notes (in Greek Religion) that in Phaistos she appears in connection with an initiation cult.

In Roman mythology her equivalent, as mother of Apollo and Diana, is Latona.

Leto was the principal goddess of Anatolian Lycia. Her sanctuary, the Letoon near Xanthos, united the Lycian confederacy of city-states. The people of Cos also claimed Leto as their own.

A measure of what a primal goddess Leto was can be recognized in her Titan father, whose name "Coeus" links him to the sphere of heaven from pole to pole, and her mother "Phoebe," who is precisely the "pure" and "purifying" epithet of the full moon.

When Hera, the most conservative of goddesses — for she had the most to lose in changes to the order of nature — discovered that Leto was pregnant and that Zeus was the father, she realized that the offspring would cement the new order. She was powerless to stop the flow of events, but she banned Leto from giving birth on "terra firma", on the mainland, or any island at sea, or any place under the sun (Hyginus, Fabulae, 140). Some mythographers hinted that Leto came down from the land of the Hyperboreans in the guise of a she-wolf, or that she sought out the "wolf-country" of Lycia for her denning. But most accounts agree that she found the barren floating island of Delos, which was neither mainland nor a real island, and gave birth there, promising the island wealth from the worshippers who would flock to the obscure birthplace of the splendid god who was to come. The island was surrounded by swans. As a gesture of gratitude, Delos was secured with four pillars and later became sacred to Apollo.

It is remarkable that Leto brought forth Artemis, the elder twin, without struggle or pain, as if she were merely revealing another manifestation of her self. For Apollo, Leto labored for nine nights and nine days, according to the Homeric Hymn to Delian Apollo in the presence of all the first among the deathless goddesses as witnesses: Dione, Rhea, Ichnaea and Themis and the sea-goddess "loud-moaning" Amphitrite. Only Hera kept apart, and perhaps she kidnapped Eileithyia or Ilithyia, the goddess of childbirth, to prevent Leto from going into labor, but Artemis was born first and then assisted with the birth of Apollo. Another version states that Artemis was born one day before Apollo, on the island of Ortygia (who is probably an ancient name of Delos), and that she helped Leto cross the sea to Delos the next day to give birth to Apollo.

Leto was threatened and assailed in her wanderings by chthonic monsters of the ancient earth and old ways, and these became the enemies of Apollo and Artemis. One was the Titan Tityos, a phallic being who grew so vast that he split his mother's womb and had to be carried to term by Gaia herself. He attempted to waylay Leto near Delphi, but was laid low by the arrows or Apollo— or possibly Artemis, as another myth-teller recalled.

Another ancient earth creature that had to be overcome was the dragon Pytho or Python which lived in a cleft of the mother-rock beneath Delphi, beside the Castalian Spring. Apollo slew it but had to do penance and be cleansed afterwards, since Python was a child of Gaia. Sometimes the slaying was said to be because Python had attempted to rape Leto while pregnant with Apollo and Artemis, but one way or another, it was necessary that the ancient oracle pass to the protection of the new god. ......