The differences between the Sunni and Shiite Islamic sects are rooted in disagreements over the succession to the Prophet Muhammad, who died in 632 AD, and over the nature of leadership in the Muslim community. The historic debate centered on whether to award leadership to a qualified, pious individual who would follow the customs of the Prophet or to transmit leadership exclusively through the Prophet's bloodline. The question was settled initially when community leaders elected a companion of the Prophet's named Abu Bakr to become the first c aliph (Arabic for "successor"). Although most Muslims accepted this decision, some supported the candidacy of Ali ibn Abi Talib, the Prophet's cousin and son-in-law, husband of the Prophet's daughter Fatima. Ali had played a prominent role during the Prophet's lifetime, but he lacked seniority within the Arabian tribal system and was bypassed. This situation was unacceptable to some of Ali's followers, who considered Abu Bakr and the two succeeding caliphs (Umar and Uthman) to be illegitimate. Ali's followers believed that the Prophet Muhammad himself had named Ali as successor and that the status quo was a violation of divine order. A few of Ali's partisans orchestrated the murder of the third caliph Uthman in 656 AD, and Ali was named caliph. Ali, in turn, was assassinated in 661 AD, and his son Hussein (680 AD) died in battle against forces of the Sunni caliph. Ali's eldest son Hassan (d. 670 AD) is also revered by Shiite Muslims, some of who claim he was poisoned by the Sunni caliph Muawiyah. Those who supported Ali's ascendancy became later known as "Shi'a," a word stemming from the term " shi ' at Ali," meaning "supporters" or "helpers of Ali." Others respected and accepted the legitimacy of his caliphate but opposed political succession based on bloodline to the Prophet. This group, who constituted the majority of Muslims, came to be known in time as "Sunni," meaning "followers of [the Prophet's] customs [ sunna ]." The caliphate declined as a religious and political institution after the 13 th century, although the term "caliph" continued to be used by some Muslim leaders until it was abolished in 1924 by Turkey's first President Mustafa Kemal Ataturk. The decline and abolition of the caliphate became a powerful religious and political symbol to some Sunni Islamist activists during the 19 th and 20 th centuries. These activists argued that leaders in the Islamic world had undermined the caliphate by abandoning the "true path" of Islam. Inspired by these figures, some contemporary Sunni Islamist extremists, such as Osama bin Laden and others, advocate the restoration of a new caliphate based on "pure" Islamic principles. The religious, ethnic, linguistic, and socioeconomic diversity that exists within the global Muslim community presents significant challenges to the reemergence of centralized, pan-sectarian, and widely recognized Islamic religious leadership. Islamic theology and sectarian considerations are rarely sufficient explanations for instances of terrorism and political violence involving Muslims or taking place in the contemporary Muslim world. Political, social, and economic factors often determine whether a given dispute reflects sectarian identities or transcends them. The use of violence by members of a given religious sect may be motivated by secular political goals or individual factors. Sunni and Shiite organizations and governments often collaborate when they perceive that their interests overlap. In other instances, theological differences can directly fuel sectarian hatred and violence and undermine calls for cross-sectarian cooperation. Members and supporters of terrorist organizations like Al Qaeda and its affiliates exhibit regional and theological diversity that makes it difficult to identify universally shared motives that can be linked to specific religious doctrines. However, many Sunni and Shiite Muslims refer to members and supporters of Al Qaeda and similar groups simply as takfiris (Arabic for "those who accuse others of apostasy") because of Al Qaeda supporters' habit of denouncing Muslim and non-Muslim individuals who don't accept their narrow interpretation of Sunni Islam as non-believers and legitimate targets. Although there are considerable differences between Sunni and Shiite Islam, the two Islamic sects share common traditions, beliefs, and doctrines. All Muslims believe that the Prophet Muhammad was the messenger of Allah (the Arabic word for God). All believe that they must abide by the revelations given to the Prophet by Allah (as recorded in the Quran) and by the hadith (sayings of the Prophet and his companions). The concepts of piety, striving for goodness, and social justice are fundamental to Islamic belief and practice. Additionally, all Muslims are expected to live in accordance with the five pillars of Islam: (1) shahada --recital of the creed "There is no God but Allah, and Muhammad is His Prophet"; (2) salat --five obligatory prayers in a day; (3) zakat --giving alms to the poor; (4) sawm --fasting from sunrise to sunset during the month of Ramadan; and (5) hajj --making a pilgrimage to Mecca once during a lifetime if one is physically and financially able. The basic sources for Islamic jurisprudence, be it Sunni or Shiite, are the Quran, the sunna (customs of the Prophet Muhammad) as relayed in the hadith (collected accounts of the Prophets sayings), qiyas (interpretive analogy), ijma ' (scholarly consensus), and ijtihad (individual reasoning). The primary function of learned religious leaders in the Islamic faith is the interpretation of Islamic law ( shari ' a ). There are no codified laws in either Sunni or Shiite Islam. Rather, there are sources for the interpretation of law outlined above, and these sources are similar among Shiites and Sunnis. Shiite hadith differ from Sunni hadith , mainly in that they include the sayings of the Shiite imams who are considered to have been divinely inspired. Shiite legal interpretation also allows more space for human reasoning than Sunni interpretation does. The majority of Muslims today are Sunnis. They accept the first four caliphs (including Ali) as the "rightly guided" rulers who followed the Prophet. In theory, Sunnis believe that the leader ( imam ) of the Muslim community should be selected on the basis of communal consensus, on the existing political order, and on a leader's individual merits. This premise has been inconsistently practiced within the Sunni Muslim community throughout history. Sunni Muslims do not bestow upon human beings the exalted status given only to prophets in the Quran, in contrast to the Shiite veneration of imams. Sunnis have a less elaborate and arguably less powerful religious hierarchy than Shiites. In contrast to Shiites, Sunni religious teachers historically have been under state control. At the same time, Sunni Islam tends to be more flexible in allowing lay persons to serve as prayer leaders and preachers. In their day-to-day practices, Sunnis and Shiites exhibit subtle differences in the performance of their obligatory prayers. Both groups share a similar understanding of basic Islamic beliefs. Within Sunni Islam, there are four schools of jurisprudence that offer alternative interpretations of legal decisions affecting the lives of Muslims. The four schools of jurisprudence rely mostly on analogy as a way to formulate legal rulings, and they also give different weight to the sayings of the Prophet and his companions ( hadith ) within their decisions. In some secular countries, such as Turkey, the opinions issued by religious scholars represent moral and social guidelines for how Muslims should practice their religion and are not considered legally binding. The four legal schools, which vary on certain issues from strict to broad legal interpretations, are the (1) Hanafi: this is the oldest school of law. It was founded in Iraq by Abu Hanifa (d. 767 AD). It is prevalent in Turkey, Central Asia, the Balkans, Iraq, Syria, Lebanon, Jordan, Afghanistan, Pakistan, India, and Bangladesh; (2) Maliki: this was founded in the Arabian Peninsula by Malik ibn Anas (d. 795 AD). It is prevalent in North Africa, Mauritania, Kuwait, and Bahrain; (3) Shaf ' i: this school was founded by Muhammad ibn Idris al Shafi'i (d. 819 AD). It is prevalent in Egypt, Sudan, Ethiopia, Somalia, parts of Yemen, Indonesia, and Malaysia; and (4) Hanbali: this was founded by Ahmad Hanbal (d. 855). It is prevalent in Saudi Arabia, Qatar, parts of Oman, and the United Arab Emirates. Sunni Islam has had less prominent sectarian divisions than Shiite Islam. The Ibadi sect, which is centered mostly in Oman, East Africa, and in parts of Algeria, Libya, and Tunisia, has been sometimes misrepresented as a Sunni sect. Ibadi religious and political dogma generally resembles basic Sunni doctrine, although the Ibadis are neither Sunni nor Shiite. Ibadis believe strongly in the existence of a just Muslim society and argue that religious leaders should be chosen by community leaders for their knowledge and piety, without regard to race or lineage. The Sunni puritanical movements referred to as " Salafism " and " Wahhabism " have become well known in the West in recent years and are highly active in many countries around the world. "Salafism" refers to a broad subset of Sunni revivalist movements that seek to purify contemporary Islamic religious practices and societies by encouraging the application of practices and views associated with the earliest days of the Islamic faith. The world's Salafist movements hold a range of positions on political, social, and theological questions and include both politically quietist and violent extremist groups. The terms "Wahhabism" and "Wahhabi" are often applied to groups and individuals who espouse a particular brand of Salafist thought commonly associated with the religious establishment of the kingdom of Saudi Arabia. In its original context, "Wahhabism" refers to a movement founded in Arabia by the scholar Muhammad ibn Abd al Wahhab (1703-1791 AD) as an offshoot of the Hanbali school of Islamic legal interpretation. Abd al Wahhab encouraged a return to the orthodox practice of the "fundamentals" of Islam, as embodied in the Quran and in the life of the Prophet Muhammad. In the 18 th century, Muhammad ibn Saud, founder of the modern-day Saudi dynasty, formed an alliance with Abd al Wahhab and unified the disparate tribes in the Arabian Peninsula. From that point forward, there has been a close relationship between the Saudi ruling family and the local Wahhabi religious establishment. The most conservative interpretations of Wahhabist Sunni Islam view Shiites and other non-Wahhabi Muslims as dissident heretics. Following the 1979 Soviet invasion of Afghanistan and Shiite Islamic revolution in Iran, Saudi Arabia's ruling Sunni royal family began allowing their clerics and citizens to more actively promote Saudi religious doctrine abroad, and Saudi individuals and organizations since have financed the construction of mosques, religious schools, and Islamic centers in dozens of countries. The content of Saudi-funded religious programs ranges from apolitical to activist depending on its sources and sponsors within the kingdom. However, in host societies many observers refer to Saudi funded or supported religious centers and clerics as "Wahhabi." Similarly, although significant differences may exist between the religious views and actions of Saudi Arabia's domestic religious establishment and those of specific Salafists active outside of the kingdom, non-Saudi Salafis frequently are identified as "Wahhabi" by other Muslims and non-Muslims who perceive them to be ideologically similar to their Saudi counterparts or believe that they receive financial support from Saudi Arabia or other Sunni Gulf states. Initially, the Shiite movement gained a wide following in areas that now include Iraq, Iran, Yemen, and parts of Central and South Asia. In most of the world, Shiites would continue as a minority. Today, according to some estimates, Shiite Islam is practiced among approximately 10% to 15% of the world's Muslim population. For Shiites, the first true leader of the Muslim community is Ali, who is considered an imam , a term used among Shiites not only to indicate leadership abilities but also to signify blood relations to the Prophet Muhammad. As Ali's descendants took over leadership of the Shiite community, the functions of an imam became more clearly defined. Each imam chose a successor and, according to Shiite beliefs, he passed down a type of spiritual knowledge to the next leader. Imams served as both spiritual and political leaders. But as Shiites increasingly lost their political battles with Sunni Muslim rulers, imams focused on developing a spirituality that would serve as the core of Shiite religious practices and beliefs. Shiites believe that when the line of imams descended from Ali ended, religious leaders, known as mujtahid s, gained the right to interpret religious, mystical, and legal knowledge to the broader community. The most learned among these teachers are known as ayatollah s (lit. the "sign of God"). Shiite religious practice centers around the remembrance of Ali's younger son, Hussein, who was martyred near the town of Karbala in Iraq by Sunni forces in 680. His death is commemorated each year on the 10 th day of the Islamic month of Muharram in a somber and sometimes violent ritualistic remembrance known as "Ashura," marked among some Shiites by the ritual of self-flagellation. As a minority that was often persecuted by Sunnis, Shiites found solace in the Ashura ritual, the telling of the martyrdom of Hussein and the moral lessons to be learned from it, which reinforced Shiite religious traditions and practices. Twelver Shiism--the most common form of Shiism today--is pervasive in Iran, Iraq, Lebanon, and Bahrain. Twelvers accept a line of 12 infallible imams descendent from Ali and believe them to have been divinely appointed from birth. The 12 imams are viewed as harbors of the faith and as the designated interpreters of law and theology. Twelvers believe that the 12 th and last of these imams "disappeared" in the late ninth century. This "hidden imam" is expected to return to lead the community. Following the 12 th imam's disappearance, as one scholar notes, a "pacifist" trend emerged among Twelvers who "chose to withdraw from politics and quietly await his coming." In the 20 th century, changes in the political landscape of the Middle East led to a new competing "activist" trend among Twelver groups in Iran and Lebanon, typified by the late Iranian religious leader Ayatollah Khomeini. Although most Shiites agree on the basic premise that Ali was the first rightful imam, they disagree on his successors. The Ismailis, who are the second-largest Shiite sect, broke off in the eighth century, recognizing only the first seven imams (the seventh was named Ismail, hence the names "Ismaili" and "Sevener"). Historically and at least until the 16 th century, the Ismailis were far more disposed than the Twelvers to pursuing military and territorial power. In the past, they established powerful ruling states, which played significant roles in the development of Islamic history. Today, Ismailis are scattered throughout the world but are prominent in Afghanistan (under the Naderi clan), in India, and in Pakistan. There are also Ismaili communities in East and South Africa. The Zaydis , who acknowledge the first five imams and differ over the identity of the fifth, are a minority sect of Shiite Islam, mostly found in Yemen. The Zaydis reject the concepts of the imams' infallibility and of a "hidden imam." Other sects, such as the Alawites and Druzes, are generally considered to be derived from Shiite Islam, although their religious practices are secretive, and some do not regard their adherents as Muslims. Alawites exist mostly in Syria and Lebanon. The Asad family that effectively has ruled Syria since 1971 are Alawite. Many Alawites interpret the pillars (duties) of Islam as symbolic rather than applied, and celebrate an eclectic group of Christian and Islamic holidays. In Turkey, the Alevi s are an offshoot group of Shiite Islam that has been often confused with Syrian Alawites or other Shiites. Most Alevis are well-integrated into Turkish society and speak both Turkish and Kurdish. The Druze community was an 11 th -century offshoot of Ismaili Shiite Islam and is concentrated in Lebanon, Jordan, Syria, and Israel. Today, the Druze faith differs considerably from mainstream Shiite Islam.

Summary:
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 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 been the basis for religious intolerance, political infighting, and sectarian violence. This report includes a historical background of the Sunni-Shiite split and discusses the differences in religious beliefs and practices between and within each Islamic sect as well as their similarities. The report also relates Sunni and Shiite religious beliefs to discussions of terrorism and sectarian violence that may be of interest to Congress. Also see CRS Report RS21695, The Islamic Traditions of Wahhabism and Salafiyya, by [author name scrubbed].