Io was an Argive princess and the daughter of Inachus, an ancient hero or river god of Argos. She also had the misfortune to be subjected to the lust of Zeus. Zeus, in an attempt to avoid the rage and jealousy of Hera, his wife, transformed Io into a handsome white heifer. Hera, who knew Zeus was up to no good, asked for the heifer as a present. Zeus could not refuse. Hera deposited Io in the safe keeping of Argus, the watchman with a hundred eyes. She was eventually rescued by Hermes, though Hera still dogged her by sending a gadfly to sting her wherever she went. This tale she eventually ended up telling to Prometheus, while he was bound to his rock. Prometheus, though he couldn't provide direct comfort, told her that, though her future would be fraught with hardship and toil, she would, upon reaching Egypt and the Nile, be restored by Zeus and bare him a son, Epaphus. Furthermore, and perhaps more importantly, she is the progenitor, thought removed by many generations, of Hercules, greatest of heroes, to whom Prometheus himself would owe his freedom.

Io, and the myth surrounding her, is important in several respects. First of all, her descendant Hercules plays such a major role in Greek mythology. Second, a number of real place names and objects are directly named for her or connected with her story. First of all Ionia, or the western coast of Asia Minor, is named after her because she reputedly ran down this coastline while she was being pursued by Hera's gadfly. Also, the Bosphorus, or Ford of the Cow, is named in memory of her passing. When Hermes rescued Io, he killed Argus, whose eyes became the tail of the peacock, a bird associated with Hera.

Finally, there are a number of common literary and mythological motifs that surround Io. First of all, Zeus' infidelity is seen here, as well as Hera's jealousy. We also find one of Zeus' most common techniques for hiding his exploits from Hera, the Long Night. While he was seducing Io, Zeus threw a cloud over the earth to hide them from Hera. This motif is also seen in connection with Alcmene and Amphitryon as found in Amphitryon by Plautus, the Roman Comedian. The story of Io is also found in Prometheus Bound, a play by Aeschylus, and Ovid.