His Highness the Aga Khan became Imam (spiritual leader) of the Shia Imami Ismaili Muslims on 11 July 1957 at the age of 20, succeeding his grandfather, Sir Sultan Mahomed Shah Aga Khan. He is the 49th hereditary Imam of the Shia Imami Ismaili Muslims and a direct descendant of the Prophet Muhammad (peace be upon him and his family) through his cousin and son-in-law, Ali, the first Imam, and his wife Fatima, the Prophet's daughter.