The study of the history of the Ismāʿīlī religion, which for long had depended largely on the polemical and often distorted accounts of its opponents, has been transformed since the time of the First World War by the discovery of large private collections of authentic Ismāʿīlī works in the Soviet Union and India. Many of the original texts, previously kept secret from outsiders by the Ismāʿīlī communities, have now been published or are accessible in manuscript to scholarly research. Although a relatively small number of scholars in the East and the West have actively pursued such research, progress in uncovering the story of the Ismāʿīlī movement in its various branches and the development of Ismāʿīlī religious thought has been steady. The major aspects and characteristics of this thought and its transformations in the course of often catastrophic events affecting the scattered Ismāʿīlī communities have become evident. There are, to be sure, still large gaps left in our knowledge of these developments, some of which may prove difficult to fill because of a lack of sources. Moreover, on some fundamental questions, especially concerning the early stages of Ismāʿīlism, consensus has not yet been reached among scholars. Yet these problems must not obscure the remarkable advances made in the study of Ismāʿīlism, which provide both a general outline of the history of one of the major branches of Shī'ī Islam and a sound basis for further detailed research.