The fourth caliph (655-661) and Muhammad's cousin and son-in-law.
Ali converted to Islam shortly after Muhammad's wife, Khadija; he was at that time a boy of eleven years old. When Muhammad emigrated to Medina in 622, Ali was chosen to stay behind and occupy his bed in order to thwart an attempt on Muhammad's life. Ali later joined the Muslims in Medina and subsequently married Muhammad's youngest daughter, Fatima; of their marriage were born two sons, Hasan and Husayn, and two daughters. He took part in almost all of Muhammad's expeditions, but after Muhammad's death he abstained for reasons unknown.
During Abu Bakr's election, Ali remained in Muhammad's house in order to prepare the funeral. He did not give allegience to Abu Bakr until six months later when his wife died; this may be due to Fatima's quarrel with Abu Bakr over her inheritance. According to Muslim chronicles, Ali was a trusted advisor of the first three caliphs on legal matters; however, with regard to administrative and political matters, Ali disagreed vehemently with his predecessors, and during Uthman's reign (644-656) he aligned himself with the opposition. His failure to punish Uthman's murderers after his accession in 656 provoked outrage. Ali was first faced with a rebellion headed by two of his former supporters and one of Muhammad's former wives, then with the refusal of the governor of Syria, Muawiya, to pledge allegiance to the new caliph. The first was ended in 656, while the second resulted in a stalemate which caused many of his supporters to abandon him; these seceders became known as Kharijites. Ali was killed by a Kharijite in 661. (see "The Four Righteous Caliphs").
Ali's supporters after Uthman's death were known as the shi`at Ali, the party of Ali, or Shi`a for short. The Shi`a revere Ali as the first rightful religious and political leader (imam) after Muhammad. Both Shi`ite and Sunni tradition portray Ali as a courageous and extremely pious man.