As many as 40 different Shi`i groups existed in the ninth and tenth centuries yet only 3 forms have survived. Why is Twelver Shi`ism one of them? As the established faith in modern Iran, the majority faith in Iraq and areas in the Gulf and with its adherents forming sizeable minorities elsewhere in the region, it is arguably the most successful branch of Shi'ism. This book charts its history and the development of the key distinctive doctrines and practices which ensured its survival in the face of repeated challenges. It argues that the key to the faith's endurance has been its ability to institutionalise responses to the changing, often localised circumstances in which the community has found itself, thereby remaining remarkably resilient in the face of both internal disagreements and external opposition.