In this trilogy there are multiple themes carried through all three plays. Other themes can be found and in one, or two, of the three plays, but are not applicable to the Trilogy as a whole and thus are not considered themes of the trilogy.
Justice through retaliation
Retaliation is seen in the Oresteia in a slippery slope form, occurring subsequently after the actions of one character to another. In the first play Agamemnon, it is mentioned how in order to shift the wind for his voyage to Troy, Agamemnon had to sacrifice his innocent daughter Iphigenia. This then caused Clytemnestra pain and eventually anger which resulted in her plotting revenge on Agamemnon. Therefore, she found a new lover Aegisthus. And when Agamemnon returned to Argos from the Trojan War, Clytemnestra killed him by stabbing him in the bathtub and would eventually inherit his throne. The death of Agamemnon thus sparks anger in Orestes and Electra and this causes them to now plot the death of their mother Clytemnestra in the next play Libation Bearers, which would be considered matricide. Through much pressure from Electra and his cousin Pylades Orestes eventually kills his mother Clytemnestra and her lover Aegisthus in "The Libation Bearers". Now after committing the matricide, Orestes is being hunted down by the Furies in the third play "The Eumenides", who wish to exact vengeance on him for this crime. And even after he gets away from them Clytemnestra's spirit comes back to rally them again so that they can kill Orestes and obtain vengeance for her. However this cycle of non-stop retaliation comes to a stop near the end of The Eumenides when Athena decides to introduce a new legal system for dealing out justice.
Justice through the law
This part of the theme of 'justice' in The Oresteia is seen really only in The Eumenides, however its presence still marks the shift in themes. After Orestes begged Athena for deliverance from 'the Erinyes,' she granted him his request in the form of a trial. It is important that Athena did not just forgive Orestes and forbid the Furies from chasing him, she intended to put him to a trial and find a just answer to the question regarding his innocence. This is the first example of proper litigation in the trilogy and illuminates the change from emotional retaliation to civilized decisions regarding alleged crimes. Instead of allowing the Furies to torture Orestes, she decided that she would have both the Furies and Orestes plead their case before she decided on the verdict. In addition, Athena set up the ground rules for how the verdict would be decided so that everything would be dealt with fairly. By Athena creating this blueprint the future of revenge-killings and the merciless hunting of the Furies would be eliminated from Greece. Once the trial concluded, Athena proclaimed the innocence of Orestes and he was set free from the Furies. The cycle of murder and revenge had come to an end while the foundation for future litigation had been laid. Aeschylus, through his jury trial, was able to create and maintain a social commentary about the limitations of revenge crimes and reiterate the importance of trials. The Oresteia, as a whole, stands as a representation of the evolution of justice in Ancient Greece.
The theme of revenge plays a large role in the Oresteia. It is easily seen as a principal motivator of the actions of almost all of the characters. It all starts in Agamemnon with Clytemnestra, who murders her husband, Agamemnon, in order to obtain vengeance for his sacrificing of their daughter, Iphigenia. The death of Cassandra, the princess of Troy, taken captive by Agamemnon in order to fill a place as a concubine, can also be seen as an act of revenge for taking another woman as well as the life of Iphigenia. Later on, in The Libation Bearers, Orestes and Electra, siblings as well as the other children of Agamemnon and Clytemnestra, plot to kill their mother and succeed in doing so due to their desire to avenge their father's death. The Eumenides is the last play in which the Furies, who are in fact the goddesses of vengeance, seek to take revenge on Orestes for the murder of his mother. It is also in this part of the trilogy that it is discovered that the god Apollo played a part in the act of vengeance toward Clytemnestra through Orestes. The cycle of revenge seems to be broken when Orestes is not killed by the Furies, but is instead allowed to be set free and deemed innocent by the goddess Athena. The entirety of the play's plot is dependent upon the theme of revenge, as it is the cause of almost all of the effects within the play.