Orestes is the son of Agamemnon and Clytemnestra. When Aegisthus and Clytemnestra murder his father, Orestes goes into exile. After several years, he journeys to the oracle ad Delphi to ask the gods' advice. The priests reply that he must kill both his mother and Aegisthus.
He then returns to avenge Agamemnon's murder. In Mycenae, he was reunited with his sister Electra. Together, the two returned to the palace and kill Clytemnestra and Aegisthus.
After the murder of his own mother, Orestes was then tormented by the Harpies. In the resulting trial for the murder, both Orestes and Electra were sentenced to death. The two fled to escape death.
Eventually, Orestes ended up in Athens. There, he was put on trial for the murder, and eventually acquitted by Athena herself. The story then becomes far more bizarre. To finally relieve himself of the Harpies, Orestes was instructed by Apollo to journey to the land of the Taurians to steal a statue of Artemis. There, he met his supposedly dead sister Iphigeneia, who had been rescued at the last minute by Artemis, and was then made her priestess for the Taurians. She aided him in his quest, and they escaped with the statue.
According to Apollodorus, he returned to become the king of Mycenae, as well as Sparta after the death of his uncle Menelaus.