Shii Muslims believe that after the death of the Prophet Muhammad in 632 ad his cousin and son-in-law Ali (d. 661) inherited Muhammad's spiritual and political authority over the umma (the Muslim community). Thus, the Shia reject the succession of the first three of Muhammad's successors (khalifa, caliph) until Ali himself became the fourth caliph (656–61). After Ali's assassination, the Shia believe that the succession lay with his male descendants, via his son al-Husayn. Each of these men is called ‘Imam’.

At present some 10–15 per cent of the world's 1 billion Muslims are estimated to be Shiites, that is between 154 million and 200 million. Today there are three main Shii groups.

Of the three, the Ismailis are the second largest. They believe that Ismail, the eldest son of the sixth Imam, Jafar al-Sadiq (d. 765), did not die but went into hiding and had a son Muhammad, who also went into hiding or died. The Ismaili community then fragmented. The Nizari Ismailis, numbering some 5–15 million adherents, are the largest remaining of the Ismaili groups and follow a living Imam: as of 2013, the Agha Khan is the forty-ninth in the line. Today, Ismailis are found in the Indian subcontinent, central Asia, the Middle East and East Africa, as well as Europe and North America.

The smallest of the three groups are the Zaydi Shia. They have their origins in those who gave allegiance to Zayd, son of the fourth Imam and half-brother of the fifth Imam. In 740, Zayd launched a revolt in Kufa against the Sunni Umayyad dynasty (661–750).