Provincial governor in the Achaemenian empire. Darius I (r. 522–486 BC) established 20 satrapies with an annual tribute. Appointed by the king, satraps were usually of the royal family or Persian nobility and held office indefinitely. They collected taxes, were the highest judicial authority, and were responsible for internal security and for raising and maintaining an army. Controls guarded against the abuse of power, but after the mid 5th century BC, with central authority weakened, satrapies became virtually independent. Alexander the Great and his successors retained the satraps.