Alcmaeonidae, a noble Athenian family, descendants of Alcmaeon, the great-grandson of Nestor. The whole family were expelled from Athens about 596 B. C. by a council of 300 nobles, to whom, by the advice of Solon, they had submitted the case of the archon Megacles, one of their number. Megacles was accused of having been guilty of sacrilege in his treachery toward Cylon and his comrades, whom he killed after promising them safety - a crime which in the opinion of the council brought a stain upon all the Alcmaeonids. After an exile of about 30 years they succeeded in returning to the city, and even, after a few years, in seizing the government and expelling Pisistratus. Although Megacles afterward restored him, and gave him his daughter in marriage, new quarrels broke out, and Pisistratus was again banished. Collecting an army, he defeated and again expelled the Alcmaeonids. During this period of renewed exile, the family magnificently restored the temple of Apollo at Delphi, which had been burned. Aided by the popularity given them by this great work, and by the friendship of the Spartans, they were in 510 restored for the second time to Athens, where many members of the family became illustrious.

Among these were Clisthenes, Pericles, and Al-cibiades.