The Musta‘lī are a branch of Isma'ilism named for their acceptance of al-Musta'li as the legitimate nineteenth Fatimid caliph and legitimate successor to his father, al-Mustansir Billah. In contrast, the Nizari—the other living branch of Ismailism, presently led by Aga Khan IV—believe the nineteenth caliph was al-Musta'li's elder brother, Nizar. Isma'ilism is a branch of Shia Islam.

Isma'ilism is a branch or sect of Shia Islam. The Isma'ili get their name from their acceptance of Imam Isma'il ibn Jafar as the appointed spiritual successor (imām) to Ja'far al-Sadiq, wherein they differ from the Twelver Shia, who accept Musa al-Kadhim, the younger brother of Isma'il, as the true Imām.