This is an Oxford doctoral thesis, published without substantial revision. One of the aims of the book is to argue that Leo ‘was the first major theologian to attempt to develop a doctrine of salvation for the changed circumstances of a Christianised Empire’, which he did by devising ‘a version of Christianity for the baptised citizen …, absorbing the virtues and duties of Roman civic life into the Gospel message’ (p. 93). This claim would need a much fuller discussion of both Leo and his predecessors than is offered here, and is in any case implausible: it is contradicted by the guidance that Leo laid down for penitents, in which he discouraged both government service and engagement in trade (ep. clxvii). The main emphasis in the book, however, is on the very different theme of Leo's Christology. Leo's debt to his Latin forbears, particularly in his use of the una persona formula, is treated with both clarity and precision. This is followed by the most substantial section of the book, which takes us through Leo's sermons in chronological order; the result is a really useful analysis, which makes this book the best introduction in English to this material, even if (inevitably) some of the exegesis could be contested. There follows a lengthy discussion of Leo's Tome. Green advances the novel theory that the Tome was as concerned to refute Nestorius as to refute Eutyches, and that its one-sided emphasis on the duality of natures in Christ arose from a misunderstanding of Nestorius: Leo thought that Nestorius accorded Christ only one nature, the human one. As result of Chalcedon, however (Green continues), Leo came to see his mistake: Nestorius' error was not a denial of Christ's divinity but an inability to provide a satisfactory account of the union of the natures. Accordingly, Leo adopted a new emphasis on unity in Christ, as we find in his ep. cxxiv to the monks of Palestine, dating to 453, the text with which Green concludes (oddly he makes no mention of Leo's later writings, such as the Second Tome). Is any of this plausible? It is certainly true that Leo came to appreciate that the Tome had backfired and that his later writings present a more balanced Christology. But I am not persuaded that the Tome was directed against Nestorianism, nor can I perceive any change in Leo's understanding of Nestorius. For the charges of psilanthropism and of splitting Christ into two Sons did not reflect two different readings of Nestorius: it was because he failed to unite the two natures in Christ that he was accused of treating Jesus as a mere man. In all, this is a well-presented doctoral thesis that needed, however, further research and discussion with other scholars before it could become the magisterial book that Green is surely capable of writing. One can only hope that this book marks the beginning rather than the end of his study of this important topic.