From the time that Twelver Shia Islam emerged as a distinct religious denomination in the early ninth century, its clergy, or ulama, have played a prominent role in the development of its scholarly and legal tradition however, the development of a distinct hierarchy among the Shia clergy dates back only to the early nineteenth century. Since that time the highest religious authority has been vested in the mujtahids, scholars who by virtue of their erudition in the science of religion (the Quran, the traditions of Muhammad and the imams, jurisprudence, and theology) and their attested ability to decide points of religious conduct, act as leaders of their community in matters concerning the particulars of religious duties. Lay Shias and lesser members of the clergy who lack such proficiency are expected to follow mujtahids in all matters pertaining to religion, but each believer is free to follow any mujtahid he chooses. Since the mid-nineteenth century it has been common for several mujtahids concurrently to attain prominence and to attract large followings. During the twentieth century, such mujtahids have been accorded the title of ayatollah. Occasionally an ayatollah achieves almost universal authority among Shias and is given the title of ayatollah ol ozma, or grand ayatollah. Such authority was attained by as many as seven mujtahids simultaneously, including Ayatollah Khomeini, in the late 1970s.

To become a mujtahid, it is necessary to complete a rigorous and lengthy course of religious studies in one of the prestigious madrasehs of Qom or Mashhad in Iran or An Najaf in Iraq and to receive an authorization from a qualified mujtahid. Of equal importance is either the explicit or the tacit recognition of a cleric as a mujtahid by laymen and scholars in the Shia community. There is no set time for studying a particular subject, but serious preparation to become a mujtahid normally requires fifteen years to master the religious subjects deemed essential. It is uncommon for any student to attain the status of mujtahid before the age of thirty more commonly students are between forty and fifty years old when they achieve this distinction.

Most seminary students do not complete the full curriculum of studies to become mujtahids. Those who leave the madrasehs after completing the primary level can serve as prayer leaders, village mullahs, local shrine administrators, and other religious functionaries. Those who leave after completing the second level become preachers in town and city mosques. Students in the third level of study are those preparing to become mujtahids. The advanced students at this level are generally accorded the title of hojjatoleslam when they have completed all their studies.

The Shia clergy in Iran wear a white turban and an aba, a loose, sleeveless brown cloak, open in front. A sayyid, who is a clergyman descended from Muhammad, wears a black turban and a black aba.

Data as of December 1987