A marja is the highest authority on religion and law in Shiism. Where a difference in opinion exist between the Marjah, Aalims (Religious Scholar) try to provide different opinions. Four senior Grand Ayatollahs [Ayat Allah] constitute the Religious Institution (al-Hawzah al-`Ilmiyyah) in Najaf, the preeminent seminary center for the training of Shiite clergymen.

Taqlid means acting according to the opinion of the jurist (mujtahid) who has all the necessary qualification to be emulated. So you do what the mujtahid's expert opinion says you should do, and refrain from what his expert opinion says you should refrain from, without any research [in Islamic sources] on your part. It is as though you have placed the responsibility of your deeds squarely on his shoulders. Among the conditions which must be found in a jurist (mujtahid) who can be followed is that he must be the most learned (al-a'lam) jurist of his time and the most capable in deriving the religious laws from the appropriate sources.

There are generally six ranks among Shi'ite clerics. The highest, grand ayatollah means "great sign of God". In the past, there were usually no more than five grand ayatollahs in the Shi'ite Islamic world. Today however it is suspected that there are at least seven and possibly more. Under grand ayatollah is ayatollah ("sign of God"). Below ayatollah is the rank of hojat al Islam, which is Arabic for "authority on Islam". Next is mubellegh al risala or "carrier of the message". While mujtahid often refers to clerics in general, it is also a specific rank, which denotes one has graduated from a religious seminary. At the bottom of the ladder are religious students, talib ilm. Besides the obvious factors such as graduation to be promoted to mujtahid, promotion in the ranks is a rather subjective matter. Two important factors behind promotion are the size and quality of one's student following and authorship of scholarly works on Islam.