It is well known, to those interested in the history of Catholic-Orthodox relations, that one of the key factors leading up to the famous excommunication of patriarch Michael Keroularios by Cardinal Humbert in 1054 was Leo of Ohrid's letter to John of Trani, which John was requested to transmit to the pope and other bishops of the ‘Franks’. This letter condemned the Roman church in no uncertain terms for its ‘Judaizing’, un-Christian use of ‘azymes’ (unleavened bread) in the Eucharist. The letter was translated by Humbert, its contents causing great scandal, and received a point-by-point rebuttal from Humbert in a piece known as the Dialogue of a Roman and Constantinopolitan, where the words of the ‘Constantinopolitan’ are simply the words of Leo's letter. Humbert's translation implicated Michael Keroularios himself as a co-author. The accuracy of this attribution to Keroularios has been much disputed: Keroularios himself denied any involvement, but while he may not have had a part in the composition of the letter, his condemnation of the use of ‘azymes’ is elsewhere quite clear. Leo's letter and its rebuttal became part of the standard ‘dossier’ on 1054, giving Leo of Ohrid lasting notoriety and fame.

There are, however, many things both about 1054 and about Leo himself which are far from well known. With regard to the events of 1054, the documentation available is considerable, of an unusual density and authenticity for the 11^th century; but despite its claims to attention, on a wide range of levels, it has not enjoyed enough popularity to merit modern editions or translations, and awareness of the details has correspondingly fallen away. Elmar Büttner's work on Leo of Ohrid therefore stands out as much-needed contribution to an area which cries out for renewed attention. With regard to Leo himself, moreover, Büttner's work makes available for the first time the range of his extant writings, with comprehensive critical apparatus, at the same time providing extensive background information to Leo and to his context.

The texts edited in this volume bring together Leo's well-known ‘First letter on the azymes’ with two further letters, on the same subject but approaching the question from different angles. There has previously been very little awareness of these further letters (the 19^th-century edition of them is highly inaccessible), which shed considerable light on Leo's thought and the connections between his ideas and those of his contemporaries. In addition, moreover, Büttner provides an edition of a previously unedited work of Leo, namely his fifty ‘Ascetic Chapters’ or ‘Kephalaia’. This latter piece follows a pattern of composition familiar from Byzantine theological texts (another of the protagonists of 1054, Niketas Stethatos, also wrote ‘Kephalaia’), but is somewhat unusual for the period in that its impact is not confined to a monastic audience but clearly envisages the laity as well. Büttner provides comprehensive manuscript information, editions and commentary for all four works, together with translations into German.

Büttner's work, however, is more than just an edition and translation. His introductory material deals in detail with Leo's life and context, bringing together the available information, offering level-headed commentary on key questions such as his relationship with Michael Keroularios and other figures and developments related to the excommunications of 1054, and providing very useful information regarding the full range of relevant texts and dates. On occasion, Büttner is able, on the basis of his investigations, to shed new light on the dynamics at play in these years. For example, much energy has, over the years, gone into discussing why Leo chose to address his first letter via John of Trani: that is, via the southern Italian territories disputed ecclesiastically between Rome and Constantinople, and at that time being dramatically politically reshaped by the Norman incursions, threatening the continued existence of any Byzantine political power in the region. That manuscript evidence in fact points also to Dominic Marango, patriarch of Grado/Venice, as a second – or perhaps even primary – conduit to the west, casts a different light on the question of Leo's method of proceeding and motivation, rendering much standard secondary comment on the subject inadequate if not inaccurate as a result.

But the interest value of Büttner's work on Leo is not confined to the events of 1054. The eleventh century is a fascinating period in Byzantine theological and ecclesiastical developments. It is probably best known for its association with Symeon the New Theologian; but Symeon represents but one trend, and a rather idiosyncratic one at that, in a very complex world, which produced many significant personalities and has left behind copious and varied written sources. Although many of the major protagonists are well known – figures such as Keroularios himself, Peter of Antioch, Niketas Stethatos (Symeon's disciple and biographer), Theophylact of Ohrid, John of Antioch – the extent and scope of their writings is often not appreciated, and often the range of approaches represented in their writings, and the tensions between them, is not appreciated. Moreover, they are the cream on the top of a great body of lesser-known material, at times more mundane but, when studied in depth, capable of presenting a much more interesting picture of the richness of the period. Much of this material is only gradually becoming more well known and widely available, through projects such as the translations of Byzantine monastic foundation documents, published online by Dumbarton Oaks, the Belfast Evergetis project, and current work in Germany on Nikon of the Holy Mountain, the key source for 11^th century Palestine and surrounding areas, to name but a few. Publication of Leo's ‘Kephalaia’ therefore is an important contribution to building up understanding of the rich complexity of this period.

Unfortunately, as yet Büttner's work is not widely available. The current publication, which I acquired through personal submission to Dr Büttner's supervisor, Professor Günter Prinzing, is not on general release, although can be acquired. It is much to be hoped that it will receive the attention it deserves and see a further edition, more widely available.