Shia Islam (Shī‘a Arabic: شيعة; ), is the second largest denomination of the Islamic Faith, after Sunni Islam, making up 10-20% of Muslims worldwide — almost all of the rest of Muslims being Sunni. Shias adhere to the teachings of Islamic prophet Muhammad but differ from Sunni in following the religious guidance of his family (who are referred to as the Ahl al-Bayt) or his descendants known as Shi'a Imams, whom they consider to be infallible. Unlike Sunnis, Shias believe Ali ibn Abi Talib (Muhammad's cousin and husband of Fatimah) was the true successor to Muhammad and reject the legitimacy of the first three caliphs of Islamic history. The singular/adjective form is (شيعي.) and refers to a follower of the faction of Ali according to the Shia ideology.

Shia Islam has at times been divided into many branches, however only three of these currently have a significant number of followers. By far the largest and most well known is the Twelvers (اثنا عشرية ) which form a majority of the population in Iran (approx. 90%) and Iraq (approx. 65%). Other branches include the Ismaili or Sevener, and Zaidiyyah. In addition, Alawites and Druzes consider themselves Shias, although this is sometimes disputed by mainstream Shias blank">Syria’s Alawis and Shi‘ism . The _Sufi orders among the Shias are the Alevi, Bektashi, Kubrawiya, Noorbakhshi, Oveyssi, Qizilbashi, Hamadani and Fatimid orders and denominations.