The official and most widely practiced religion in Iran is Shi'ism, which is a branch of Islam. Iran is a constitutional theocracy, which means that the principles and codes of one particular religion, in this case Shi'ism, have become the laws of the state. Some Shi'ite rules, including those regarding women, are much stricter than in other forms of Islam, such as Sunni. In addition, mullahs (clergy) play a much more central role in Shi'ism than in other varieties of Islam.

Islam was founded in the 7th century by Mohammed, who claimed to be the last in a line of prophets that includes Moses, Abraham and Jesus. Islam is based on the Koran, the holy book of Mohammed's teachings, and gained wide popularity in Iran in the 7th and 8th centuries. After Mohammed's death, a split over the question of religious leadership created two main branches or sects of Islam: Sunni and Shi'ism. In Shi'ism, an imam, such as Ayatollah Khomeini, is the holy leader of the Shi'ite community, whereas for Sunni Muslims, an imam is simply a man who leads people in prayer.

In common with Sunni Muslims, Shi'ite Muslims follow the five pillars of Islam, but add two others. The pillars are professing the faith, which dictates that Allah is the one God and Mohammed his prophet; praying daily; giving alms to the poor; observing the fast of Ramadan; and making the hajj or pilgrimage to the holy city of Mecca at least once in a lifetime, if possible. The two other Shi'ite pillars are jihad, the crusade to protect Islamic beliefs, and the requirement to do good works and avoid all evil.

A small number of Iranians, mostly Kurds, practise Sunni Islam. Iran's diverse ethnic population also includes followers of other religions, although these constitute only about one per cent of the population. The central region of Iran is still home to Zoroastrians, followers of the ancient, pre-Islamic state religion of Iran. Other areas have communities of Christians, Jews and Baha'is. With the exception of the Baha'i, followers of all religions are officially protected under Iran's current constitution.

  Did you know?
Zoroastrians do not bury their dead, but leave them on the "towers of silence" to be consumed by vultures. This practise is based on the belief that all elements are sacred and pure, and thus both the earth and air would be polluted by burial or cremation.

  Did you know?
Iranian Shi'ites recognize 12 imams as direct successors of Mohammed. Only these imams have been permitted to interpret the Koran.