Ali al-Husayni al-Sistani is an Iranian-Iraqi Islamic scholar. One of the most senior scholars of Twelver Shia with the rank of Grand Ayatollah and marja', he has been described as the spiritual leader of Shia Muslims worldwide, "the undisputed leader of Iraq's Shias", included in top positions of "The Muslim 500: The World's Most Influential Muslims", from 2009 to 2023, and named one of the 100 most influential people in the world by Time magazine in 2004 and 2005.

Twelver Shīʿism, also known as Imāmiyya, is the largest branch of Shīʿa Islam, comprising about 85 percent of all Shīʿa Muslims. The term Twelver refers to its adherents' belief in twelve divinely ordained leaders, known as the Twelve Imams, and their belief that the last Imam, Imam al-Mahdi, lives in Occultation and will reappear as the promised Mahdi.