This careful and scholarly study assembles and discusses the available evidence for the ecllesiastical organisation of the Church of the East (the so-called 'Nestorian' church) in the Middle East between the fourteenth and twentieth centuries. The author has built on the work of the late J.M. Fiey, but has covered a wider geographical area and used a much wider range of sources. Besides drawing on the memoirs of European and American missionaries and other literary sources, the author has consulted a large number of manuscript catalogues, many of which are only accessible in Arabic sources, and has analysed the evidence of more than 2.500 East Syrian manuscript colophons to establish the dioceses of the Church of the East at different periods, to identify its ecclesiastical elites (patriarchs, bishops, priests, deacons and scribes), and to analyse the rivalry between the church's traditionalist and Catholic wings after the schism of 1552. The study contains a number of detailed maps, which localise hundreds of East Syrian villages in Kurdistan, and will be an indispensable reference tool for scholars of the Church of the East.