Thyestes felt he had been deprived of the Mycenean throne unfairly by his brother, Atreus. The two battled back and forth several times. In addition, Thyestes had an affair with Atreus' wife, Aerope. In revenge, Atreus killed Thyestes' sons and served them to him unknowingly. After eating his own sons' corpses, Thyestes asked an oracle how best to gain revenge. The advice was to father a son with his own daughter, Pelopia, and that son would kill Atreus.
When Aegisthus was born, his mother was ashamed of her incestuous act. She abandoned him and he was raised by shepherds and suckled by a goat, whence his name Aegisthus (from αίξ). Atreus, not knowing the baby's origin, took Aegisthus in and raised him as his own son.
In the night in which Pelopia had shared the bed of her father, she had taken from him his sword which she afterwards gave to Aegisthus. This sword became the means by which the incestuous intercourse between her and her father was discovered, whereupon she put an end to her own life. Atreus in his enmity towards his brother sent Aegisthus to kill him; but the sword which Aegisthus carried was the cause of the recognition between Thyestes and his son, and the latter returned and slew his uncle Atreus, while he was offering a sacrifice on the seacoast. Aegisthus and his father now took possession of their lawful inheritance from which they had been expelled by Atreus.
Aegisthus and Thyestes thereafter ruled over Mycenae jointly, exiling Atreus' sons, Agamemnon and Menelaus to Sparta, where King Tyndareus gave the pair his daughters, Clytemnestra and Helen, to take as wives.
Homer appears to know nothing of all these tragic occurrences, and we learn from him only that, after the death of Thyestes, Aegisthus ruled as king at Mycenae and took no part in the Trojan expedition. While Agamemnon, the son of Atreus, was absent on his expedition against Troy, Aegisthus seduced Clytemnestra, the wife of Agamemnon, and was so wicked as to offer up thanks to the gods for the success with which his criminal exertions were crowned. In order not to be surprised by the return of Agamemnon, he sent out spies, and when Agamemnon came, Aegisthus invited him to a repast at which he had him treacherously murdered. After this event Aegisthus reigned seven years longer over Mycenae, until in the eighth Orestes, the son of Agamemnon, returned home and avenged the death of his father by putting the adulterer to death.