Diomedes is the son of Tydeus, one of the Seven against Thebes. Diomedes, along with the other Epigoni, attacked Thebes and destroyed it.
During the Trojan War, Diomedes is the leader of the forces from Argos and Tiryn. He fought very bravely. In book 5 of the Iliad, Diomedes, although wounded by an arrow, kills many Trojans, and wounds the great Aeneas. Aeneas would have died, except that his mother Aphrodite saves him. As she is carrying him away, Diomedes wounds the goddess herself in the wrist with his spear. She drops her son Aeneas, but Apollo saves him from the battle. Hurting, Aphrodite flees back to Olympus with the help of Iris and Ares.
In book 8, Diomedes comes to the aid of the old Nestor, when his horses are killed. Together, the two nearly kill Hector, who is saved by Zeus himself. Later, Diomedes accompanies Odysseus in a nighttime expedition to Troy, to fight out about the Trojan plans. Together, they captured Dolon, a Trojan spy. In an attempt to save his life, he tells them about Rhesus and his horses. They then kill him. Then, they raid the camp of the Trojan ally Rhesus, and they steal his famous horses.
After the war, Diomedes returned home safely. According to the Aeneid, Diomedes ended up in Italy, settling in Apulia.