Medusa was the only mortal sister of the three Gorgons, who were transformed into monstrous Gorgons by Athena after defiling her sanctuary. Medusa, also the most well-known of the three sisters, had snakes for hair and was hideously ugly. If ever anyone was to look into her eyes, they would instantly turn into stone.
Before being transformed into a Gorgon, Medusa had lay with Poseidon and conceived Pegasus and Chrysaor, though not yet born.
In Perseus' adventures, he had gained the favors of Athena, who lent him a magical reflective shield. Using this shield, Perseus approached Medusa obliquely, rendering her stone-turning ability useless. Perseus was then able to decapitate her. In doing so, he released Pegasus and Chrysaor into the world, who flew fully grown from Medusa's neck. Perseus returned the shield to Athena, along with Medusa's head. Since then, the image of Medusa's head can be seen on Athena's shield. Often snakes can also be seen to adorn Athena's garments and head as a symbol of Athena's dominance over Medusa.