What are the main differences between Sunnism and Shi'ism, the two largest sects of Islam?

The division dates back to the time after Muhammad's death in 632, in the area which is now known as Saudi Arabia, when the next leader of the Muslim nation had yet to be decided. One group of people (who would later become known as the "Shiites") believed that the ruler should be a member of the prophet's family, while another group (who would later be called the "Sunnis") believed that Muhammad’s successor should be chosen from amongst those who were most qualified. While Shiites desired the succession of Muhammad’s cousin “Ali” as the next leader, Sunnis opted for “Abu Bakr,” a close friend to the prophet. The Sunnis quickly prevailed and Abu Bakr was installed as Muhammad's successor.

Rather than ending with Abu Bakr's succession, however, the conflict between the two groups only escalated further over the following decades. In 656, a full-scale civil war engulfed the Muslim nation over the question of leadership. The war, which lasted for five years, hardened divisions between the two groups and ultimately led to a formal split of the religion into two distinct sects.

Since the split, several religious differences have emerged between the two groups. Shiites, for example, have more of a formal religious hierarchy than Sunnis do. For Shiites, those who are descendants of the prophet are particularly important and are often looked to for spiritual and social guidance. For Iraq’s Shiite population, for example, the clerics Ayatollah Sistani and Muqtada al-Sadr are prominent, influential figures because of their positions as descendants of Muhammad. In the picture, you’ll notice that both Sistani (in the banner to the left) and Sadr (right) wear black turbans around their heads, a symbol that shows lineage to the prophet.

For Sunnis, on the other hand, there does not exist the same religious hierarchy. To them, elevating descendants of the prophet to positions of power is an incorrect interpretation of the will of God. Instead, they hold the Qur'an and the teachings of Muhammad to be the primary sources of information on Islam. Shiites and Sunnis also disagree somewhat in their interpretation of certain passages of the Qur'an and the Hadith (the words and actions of Muhammad) and they also differ in the manner in which they pray.

Despite these variations, the two groups have far more similarities than differences. Both share a belief in most of the core values of Islam, including that of the Five Pillars, religious duties that make up the framework of the Islamic faith. The Five Pillars include prayer (five times a day), an annual fast during the month of Ramadan, a pilgrimage to the city of Mecca, a declaration of one's faith in God, and the regular giving of charity.

What are the demographics of Shiites and Sunnis in the Middle East?

Sunnis make up roughly 85% of the Muslim population in the world. In the Middle East, most countries have majority Sunni populations. Egypt, Saudi Arabia, the Palestinian territories, Jordan, Turkey, and the Gulf states are all Sunni-dominated.

Shiites, on the other hand, maintain majorities in Iraq, Iran, Bahrain, and Lebanon. Iran, in particular, has a very large population of Shiites. Over 60 million, in fact, which is more than in any other country in the world. In the map to the right, you'll notice that the light green Sunni-dominated areas cover far more territory than the dark green Shia-dominated areas.

In terms of power distribution, Sunnis rule most Middle Eastern countries. Shiites rule in Iran, however, and they currently dominate the Iraqi government as well. On the other hand, Shiites do not hold much power in either Bahrain or Lebanon, despite both country's Shiite majorities. Members of a sub-sect of Shi'ism (the Alawites) rule in Syria, though many Muslims do not consider the Alawites to be real Shiites.

It’s interesting to note that only about 15-20% of Muslims even live in the Middle East. The rest live primarily in countries in sub-Saharan Africa and in parts of Asia. In fact, the country with the largest Muslim population is Indonesia, which has over one hundred and eighty million Muslims.

Note: Thanks to Juan Cole and Issandr El Amrani for their important contributions to this post.