Originally the Boule was made up of 400 men, 100 from each of the four traditional tribes of Athens (the Pentacosiomedimi, Hippeis, Zeugitae and Thetes). It met on the Pnyx hill near the Acropolis. It acted as an advisory body to the Areopagus, the aristocratic council, and prepared an agenda for discussion in the lower assembly, the ecclesia. Under the reforms of Cleisthenes, the Boule was expanded to 500 men, 50 men from each of the ten new tribes of Athens. The 500 men were chosen from Athenian citizens in the ecclesia over the age of 30, and served for one year. The leaders of the Boule (the prytany) consisted of 50 men chosen from among the 500, and a new prytany was chosen every month. The man in charge of prytany was replaced every day from among the 50 members.
After the reforms of Ephialtes and Pericles in the mid-5th century BC, the Boule took on many of the functions of the Areopagus, which by this time had almost all of its powers taken away.