Today, some 85 percent of the world’s Muslims consider themselves to be Sunni Muslims, while the remaining 15 percent are adherents of the different sects of Shia Islam.

The designation Sunni Islam, or Sunnism, refers to the Prophet Muhammad’s customs (sunna). For Sunni Muslims, it is a guiding principle for each individual and for Muslim society as a whole – when confronted by new challenges – to follow the rules for living that the Prophet himself practiced. Sunni imams consult the traditions (hadith) that have been written down on the words and deeds of Muhammad when they need to find solutions to problems that are not expressly described in the Koran.

While Sunni Islam emphasizes the importance of Muhammad’s customs, Shia Islam emphasizes the special authority of the Prophet’s relatives (ahl al-bayt). Male descendants of the Prophet’s closest relative, Ali ibn Abi Talib – the Prophet’s cousin and son-in-law – are thus considered to be the ideal imams by Shia Muslims. But different views about the rightful succession led to Shiism being divided into many branches, each with its line of legitimate imams. The most important branches are the Fivers (Zaidites), the Seveners (Ismailites), and the Twelvers.

Islamic society split into its two main groups right after Muhammad’s death. In the disagreements about the rightful successor (khalifa) for the position of supreme leader of the Muslim community, the Shiites demanded that power be bestowed on Ali. The word shia is in fact an abbreviation of shiat Ali, “the party of Ali.” The other members of the Muslim community – later called the Sunni Muslims – insisted on succession in keeping with old Arab customs, which meant by election.

As a result of these disputes, the Sunni Muslim majority won and the first three caliphs – Abu Bakr, Umar, and Uthman – were elected from among the Prophet’s especially faithful adherents, and not only among his blood relatives. The fourth caliph elected, Ali, who in fact was a blood relative, ruled for five years.

When Ali died in 661, what proved to be a long and bloody struggle began between Shia and Sunni Muslims over who was to rule the Islamic world. While the fortunes of war changed from time to time, only a minority of the Islamic world’s many dynastic states have rested on a Shia Muslim foundation, however. Egypt under the Fatimids (969-1171) and Iran under the Safavids (1501-1732) are the most famous. Today Iran is the only country in the world where Shiism is the state religion.