The monarchs of Iran ruled for over two and a half millennia, beginning as early as the 8th century BC and enduring until the 20th century AD. The earliest Iranian monarch is generally considered to have been either Deioces of the Median dynasty or Cyrus the Great of the Achaemenid dynasty. The last Iranian monarch was Mohammad Reza Shah of the Pahlavi dynasty (1925–1979), who was overthrown in the Iranian Revolution. Since then, Iran has been governed by theocratic supreme leaders.