"The majority of the world's Muslim population follows the Sunni branch of Islam, and approximately 10-15% of all Muslims follow the Shiite (Shi'ite, Shi'a, Shia) branch. Shiite populations live in a number of countries, but they constitute a majority in Iran, Iraq, Bahrain, and Azerbaijan. There are also significant Shiite populations in Afghanistan, Kuwait, Lebanon, Pakistan, Saudi Arabia, Syria, and Yemen. Sunnis and Shiites share most basic religious tenets. However, their differences sometimes have served as the basis for religious intolerance, political infighting, and violent confrontations. This report includes a historical background of the Sunni-Shiite split and the differences in religious beliefs and practices between and within each Islamic sect as well as their similarities. This report will not be updated. Related CRS [Congressional Research Service] products include CRS Report RS21432 and CRS Report RS21695."
CRS Report for Congress, RS21745