Like all other major religions, Islam has been divided into several schools and denominations. The two main branches in Islam are Sunnism and Shiʿism. Sunni Muslims endorsed the historical caliphate, while Shiʿi Muslims, supporters of ‘Ali, cousin and son-in-law of the Prophet and the fourth caliph, articulated their own distinctive doctrines. The Sunni-Shiʿi schism is often framed as a dispute over the identity of the successor to Muhammad, whereas in reality, Sunni and Shiʿi Muslims also differ on a number of seminal theological doctrines concerning the nature of God and legitimate political and religious authority. Shiʿi Islam as the second largest group in the Muslim world and as the largest religious community in several countries including Iran, Iraq, Bahrain and Lebanon has gained the attention of scholars of religion and researches more than anytime in the history. The Islamic revolution in Iran in 1979 and its huge national and international geopolitical outcomes was an important factor that caused many to realize the importance of having a deeper understanding of the religious undercurrents in the Middle East in particular and in the world in general. Also, Shiʿi Islam is known today as a distinctive and vibrant community of Muslims with a remarkable capacity for reinvention and adaptation, grounded in a unique theological interpretation of Islam.