Some titles for traditional Islamic leaders include Caliph, Imam, Sheikh, Mufti, Mujtahid and Allamah. The titles of "Ayatollah" and "Grand Ayatollah" exist only in the Shiite sect of Islam.
Islamic religious leaders come in a variety of types due to the absence of an organized hierarchical structure, such as a church in Christianity. The Imam is the most popular leadership position in Islam. It denotes one who leads worship and offers advice within a community in the Sunni sect. The Shias, on the other hand, believe that Imams are descendants of Muhammad. Caliph refers to the supreme religious and political figure in a caliphate, a sovereign state encompassing the entirety of the Muslim nation, called the "Ummah." There has not been a Caliph since Abdulmecid II in 1924.