This book deals with the major Islamic movements in Iran from the time of the Arab conquest in the 7th century to the Mongol invasion in the 13th century. They range from a sect amalgamating Iranian dualist with Islamic traditions, like the Mazdakite Khurramiyya, to trends and schools of mainstream Sunnite Islam like the Murji’a, traditionalism, Hanafism and Shaf'ism, the ascetic and mystical trends of the Karramiyya and Sufism, and the religio-political opposition movements of Kharijism and Imami, Zaydi, and Isma'ili Shi'ism. The author traces the origins, development, and interaction of these movements and relates them to their specific Iranian environment in order to reveal their significance in the religious and social evolution of Iran independent of their ramifications elsewhere in the Islamic world. Special attention is paid to the socially integrative aspects of the doctrine of these religious groups and to their relations with the established governments. Much recent research and new perspectives are integrated for the first time to offer an original survey of major currents of Islam in Iran before its transformation by the Mongol conquest and the Safavid adoption of Twelver Shi’ism as the state religion.