Cousin and son-in-law of Muhammad, fourth caliph of Sunni Muslims, and first imam of Shii Muslims. First male convert to Islam and second convert after Muhammad's wife Khadijah. Married Muhammad's daughter Fatimah. Father of sons Hasan and Husayn and daughters Zaynab and Umm Kulthum. Participated in most expeditions during Muhammad's lifetime. Distinguished judge, pious believer, and brave warrior. Shiis believe that he was appointed by Muhammad as successor (the appointment is celebrated as the Festival of Ghadir on 18 Dhu al-Hijjah) and place him next to God and Muhammad as the center of religious belief. Sunnis acknowledge him only as the fourth caliph. His rule as caliph was marked by political crisis and civil strife. Assassinated while praying in a mosque in 660. Ali serves as the paradigm for political activism for redress of social and political injustices for the downtrodden. His political discourse, sermons, letters, and sayings have served as the Shii framework for Islamic government.