In Greek mythology, King Atreus (Greek: Ατρεύς, Atreús, Ατρέας ) ("fearless") of Mycenae was the son of Pelops and Hippodamia and father of Agamemnon and Menelaus. Atreus and his twin brother, Thyestes, were exiled by their father for having murdered their step-brother, Chrysippus in their desire for the throne of Olympia. They took refuge in Mycenae, where they ascended to the throne upon the absence of King Eurystheus, who was fighting the Heracleidae. Eurystheus had meant for their lordship to be temporary; it became permanent due to his death in conflict.
Entrance to the tomb grave (tholos) of Atreus ("Treasure of Atreus") built around 1250 BC at Mycenae.
Atreus vowed to sacrifice his best lamb to Artemis. Upon searching his flock, however, Atreus discovered a golden lamb which he gave to his wife, Aerope, to hide from the goddess. She gave it to her lover, Thyestes (also Atreus' brother), who then convinced Atreus to agree that whoever had the lamb should be king. Thyestes produced the lamb and claimed the throne.
Atreus retook the throne using advice he received from Hermes. Thyestes agreed to give the kingdom back when the sun moved backwards in the sky, a feat that Zeus accomplished. Atreus retook the throne and banished Thyestes.
Atreus then learned of Thyestes' and Aerope's adultery and plotted revenge. He killed Thyestes' sons and cooked them, save their hands and feet. He served Thyestes his own sons and then taunted him with their hands and feet.
An oracle then advised Thyestes that, if he had a son with his own daughter (Pelopia), that son would kill Atreus. Thyestes did so and the son, Aegisthus, did kill Atreus. However, when Aegisthus was first born, he was abandoned by his mother, ashamed of her incestuous act. A shepherd found the infant Aegisthus and gave him to Atreus, who raised him as his own son. Only as he entered adulthood did Thyestes reveal the truth to Aegisthus, that he was both father and uncle to the boy. Aegisthus then killed Atreus.
According to some sources, Atreus was the father of Plisthenes. More commonly though, they were brothers.
Apollodorus, Epitome II, 10-16; Euripides. Electra.
Tomb grave (tholos) of Atreus ("Treasure of Atreus") built around 1250 BC at Mycenae.
Atreus and Thyestes From Larry Gonick, The Cartoon History of the Universe, Volumes 1-7