Persian Sovereigns, also known as Shahs, were rulers of the Persian Empire from the 16th century to the 20th century. The term is derived from the ancient Persian word for king, and was used to refer to the monarchs of the Achaemenid, Parthian, and Sassanid dynasties. These rulers were responsible for the expansion of the Persian Empire, which at its peak stretched from India to Egypt. They were also responsible for the introduction of Zoroastrianism, the official religion of the Persian Empire.