Experts on Islam consider Kitab Firaq al-Shi'a a seminal work for two main reasons: Firstly, it is the earliest work on the subject that has survived in its entirety. Secondly, it is the earliest textbook that provides an Imami Shi'a perspective on the differences among, and origins of, the Islamic sects including a full exposition of various sects within Shi'ism. Since the German scholar, Helmut Ritter, published the first Arabic edition of the book in 1931, it has emerged as one of the important classics in its field and an indispensable tool for researchers on Islam. Properly introduced and extensively annotated, this edition is the first complete English translation of Kitab Firaq al-Shi'a. al-Hasan ibn Musa al-Nawbakhti was a tenth-century CE theologian and philosopher in Baghdad whose most important contributions were to the field of history of religions and sects. Kitab Firaq al-Shi'a is his only extant work.