Alcmaeonidae, Athenian family powerful in the 7th, 6th, and 5th cent. B.C. Blamed for the murder of the followers of Cylon, the would-be tyrant (c.632 B.C.), they were considered attainted and were exiled. They were again in Athens in the 6th cent. The most prominent members of the family later were Cleisthenes, Pericles (whose mother was an Alcmaeonid), and Alcibiades.