The ‘Disputation’, or inter-faith dialogue between Timothy i, Catholicos of the Church of the East, and the Caliph al-Mahdi is one of the most attractive of the many medieval texts dealing with Christian–Muslim disputations. What we have is a record, in the form of a letter from Timothy to a friend, of a conversation between him and the Caliph that took place over two days in 782/3. The work has deservedly proved very popular among Christians in the Middle East, and several separate translations were made into Arabic; one of the earliest witnesses to these happens to be an eleventh-century fragment in the Cairo Geniza. The Syriac text was first published (with an English translation) in 1928 by Alphonse Mingana as a facsimile of a nineteenth-century manuscript in the Mingana Collection (Birmingham). Heimgartner has been able to work from photographs of the manuscript in Baghdad, dated 1299, which is the source, direct or indirect, of all eight nineteenth-century copies that are known today. This has allowed some important improvements in the text, seeing that Mingana's copy has a number of errors. Unusually for the CSCO series, the text is set out line by line of the Baghdad manuscript. Whereas the introduction to the text volume deals more directly with matters concerning the edition, that to the translation volume covers some wider issues of the transmission of the text, including the various (sometimes adapted) Arabic translations, one of which, re-edited in 1975 by H. Putman with a French translation, has regrettably become the standard text of the Disputation cited in modern literature on the subject, to the neglect of the Syriac original. This makes Heimgartner's careful new edition and translation of the Syriac all the more welcome and significant. The rest of the introduction deals with the date of the Disputation, the form and structure of the work, and the question of whether it is a literary fiction (as has been suggested by some) or a written-up record of a genuine dialogue (as seems more probable). The annotated translation is very carefully done, and is a considerable improvement on that of Mingana, who took a number of liberties. Heimgartner's edition and translation should henceforth be taken as the authoritative source of reference for all citation of this important (and still relevant) example of Christian-Muslim dialogue.