The Ismailis represent the second largest Shia Muslim group after the Twelvers. They are spread as religious minorities in more than twenty-five countries, and most recognize the Aga Khan as their spiritual leader. This book is the first comprehensive work on the history of the Ismailis and the development of their doctrines, from their emergence in the formative period of Islam. The author conducts a detailed investigation, traversing the "golden age" of the Fatimids and the turbulent Tayyibi period, through the flourishing era of Nizari Ismailism in Iran and Syria, and ending with the massacre of the Ismailis by the Mongols. It then addresses the modern developments of the Ismaili community, explaining the revival of Nizari Ismailism, particularly in Iran and Central and South Asia, and the socio-economic progress of Nizari communities in modern times. The book contains an extensive bibliography and illustrations.