Islam is generally divided into two main branches, Sunni and Shia. Most Muslims are Sunni, but there are still as many as 200 million Shia Muslims, about ten percent of the total. Shia Muslims constitute the majority of Muslims in Iran and Iraq, and there are significant numbers of them throughout the Middle East and South Asia.

Shia Muslims believe that Ali should have been the first caliph. Sunni Muslims believe that it should have been Abu Bakr. Ali was Muhammad's son in law. Shia Muslims believe that only descendants and relatives of Muhammad could be caliph. Therefore, they opposed giving the position to Abu Bakr. In 631, Ali was murdered, and the supporters of Abu Bakr got their way with his ascension soon afterward. This was the beginning of a divide that lasts to the present day.

As a result of this first conflict, the Shia have seen themselves as the party of the persecuted minority that opposes the powerful and wealthy. To this end, there is a large social justice aspect to Shia practices and beliefs. As far as doctrine and theology are concerned, the actual differences between Shia and Sunni are rather small.

Shia clergy is generally divided into six ranks. At the top is the Grand Ayatollah. There are at least seven Grand Ayatollahs today. Seminary graduates who are approved to lead worship in formal settings are referred to as mujtahid. The lowest rank of Shia clergy is the seminary students themselves, known as talib ilm.