The Imāmī Shī'ī theory of the imāmate evolved gradually during the first Islamic century and was given a definitive shape in the middle of the second/eighth century by Hishām b. al-akam. For the next 100 years or so, until the death in 260/874 of the eleventh Imām, al-asan al-'Askarī, no significant changes seem to have been introduced. Only in the mid-fourth/tenth century does a major addition appear in the form of a doctrine: it is the belief that there are 12 Imāms, the last of whom remains in a state of concealment (ghayba) until his ultimate return as Mahdī, or Qā'im. This ghayba is divided into two periods: a shorter, ‘lesser’ ghayba (al-ghayba al-ughrā), lasting from 260/874 to 329/941, during which the Imām was represented on earth by four successive safīrs; and a longer, ‘greater’ ghayba (al-ghayba al-kubrā), whose duration is known only to God. It is this doctrine which distinguishes Twelver Shī'ism from the earlier Imāmiyya, and it io worth examining in some detail ite origina and the-main-stages of its development.