In ancient Greek mythology, Cerberus is the three-headed hound guarding the entrance to the underworld.
Cerberus is usually described as a monstrous dog belonging to Hades, the god who rules the Greek underworld. His duty was to prevent the dead from escaping and the living from entering without permission. Ancient writers do not always agree on his exact appearance: the most familiar version gives him three heads, a serpent for a tail, and snakes around his neck or body.
The hero Heracles captured Cerberus as the final of his Twelve Labors, bringing the creature temporarily out of the underworld. This episode helped make Cerberus one of the best-known figures in Greek afterlife mythology.
Cerberus is often confused with Orthrus, another monstrous dog in Greek myth. Orthrus had two heads and guarded the cattle of Geryon, whereas Cerberus guarded Hades. Later art and literature sometimes portray Cerberus with more or fewer heads, but the three-headed form became canonical.