Solon (594 BC), Cleisthenes (508/7 BC), and Ephialtes (462 BC) all contributed to the development of Athenian democracy. Historians differ on which of them was responsible for which institution, and which of them most represented a truly democratic movement. It is most usual to date Athenian democracy from Cleisthenes, since Solon's constitution fell and was replaced by the tyranny of Peisistratus, whereas Ephialtes revised Cleisthenes' constitution relatively peacefully. Hipparchus, brother of the tyrant Hippias, was killed by Harmodius and Aristogeiton, who were subsequently honored by the Athenians for their alleged restoration of Athenian freedom.
The greatest and longest lasting democratic leader was Pericles; after his death, Athenian democracy was twice briefly interrupted by oligarchic revolution towards the end of the Peloponnesian War. It was modified somewhat after it was restored under Eucleides; the most detailed accounts are of this fourth-century modification rather than the Periclean system. (wiki)