Cleisthenes (also Kleisthenes) was an Athenian politician in the latter half of the sixth century B.C.. He held the position of archon, under the tyrant Hippias in 425B.C.. However, in the latter part of Hippias' reign he and his family went into exile and bribed the Delphic Oracle to tell Sparta to invade Athens, and depose Hippias. The Spartan king Cleomenes did this, and in the power vanccum that followed Cleisthenes competed against Isagoras. Isagoras got the archonship for 508 B.C. however Cleisthenes gained popular support for his proposed democratic reforms. Isagoras appealed to Cleomenes, who returned with a small force, to drive Cleisthenes into exile. The populace of Athens was against this, and Cleomenes was forced to withdraw, and Cleisthenes returned.

Cleisthenes reorganised the political system of Athens, instituting the system of demes, trittyes, and phylai, through which all future political organisations of Athens were based. There were 139 demes grouped into either coastal, rural, or urban, membership of demes became hereditary through the male line and irrelevant to residence. The deme assembly could appoint any officials it wanted to making it in effect a miniature polis. There were 30 trittyes which consisted of one or more demes of the same type all in a neighbouring block, the number of demes in a trittyes depended on the size of the demes. There were ten phylai named for local heroes, each consisting of 3 trittyes, one coastal, one rural, and one urban, so that each phylai had similar interests rather than an urban phylai competing with a rural phylai for the distribution of public funds. The phylai became brigading units for the army and to some extent for the navy as well. The phylai also acted as constituencies for the majority of public appointments, for example each phyle would elect one of the ten generals, an office created by Cleisthenes. This underminded old channels of influence, and provided means of political power at the lower level. The Boule was a council of 500 members elected by lot for one year, each tribe would provide 50 members proportionately selected from each deme. Solon had divided Athens into 4 property classes, the five hundred bushel men, the knights, the hoplites, and the rowers, of these only the poorest, the rowers were inelligible to sit on the council. The Boule's primary job was to decide business to be brought before the Assembly, to get a topic raised a member would have to bring his request to the council who would consider it or he could use a friend on the council to raise it. The groups of 50 would each serve as prytaneis for one month. The prytaneis was the executive body of the Boule, they were on duty every day and planned meetings of the council, they also received envoys and letters of state. Every day a different member of the prytaneis was picked to be the epistates, who remained on duty for 24 hours with the members of his trittys in the prytaneis, he had the keys to the treasury and chaired any meetings of the Boule or the Assembly. The prytaneis were ordered by lot so no one phyle would always be responsible for the collection of tribute or any other important annual event. The archonship was made open to members of the top two property classes, namely the five hundred bushel men and the knights, and so the Areopagus was made open to these two classes as well. Cleisthenes kept a large amount of power with the Areopagus and so his was an aristocratic democracy until Ephialtes turned it into a radical democracy. He is also credited with the creation of ostracism.

Cleisthenes is regarded by many as the father of Athenian democracy although Solon also has claims to this title.

Cleisthenes was also the name of a tyrant of Sicyon, he is believed to have been born around 665B.C.. He gained power in 600B.C. and he is linked with a movement against the Dorian ascendancy in Argos, Dorian tribes were given insulting names, non-Dorians were referred to as the "ruling people", a Theban hero replaced an Argive one, and Argive rhapsodies were suppressed. He featured in the First Sacred War where he was responsible for the destruction of Crisa. His side eventually won the war, defeating Sicyon's neighbour Corinth. His reign came to an end in 570 B.C. although he lived on until 565B.C..

In the first ever Pythian Games at Delphi he won the chariot race. His daughter gave birth to the Athenian Cleisthenes described above.

Finally Cleisthenes was also the name of a prominent Athenian homosexual, who lived during the Peloponnesian war. Aristophanes mentions him in several of his plays, always in a derogatory fashion.