This thought-provoking study argues that, contrary to a widespread assumption, the Roman Catholic and ‘Anglican-Episcopal’ clergy have never, since the High Middle Ages, been categorically forbidden to bear arms. Its core is an analysis of changes in canon law between c. 1120 and 1317. Hitherto, the Church had rigorously forbidden the clergy to bear arms. No fewer than twelve major councils reiterated the prohibition between 1049 and 1079. After 1120, however, the law changed gradually but fundamentally. It accepted the authority of the pope and of other prelates exercising both spiritual and temporal power to declare and direct just wars; the military obligations assumed by the clergy together with lands bestowed upon them by lay donors; and every cleric's right to defend himself against attack, even to wound or kill if necessary to save his own life. Developments that helped to bring about these crucial changes included the wars against the external and internal enemies of Christianity and the Church, the foundation of military-religious orders, the formulation of the theory of the just war, the revival of Roman law, and a new emphasis on intention in both law and theology. After 1317 canon law concerning clerical arms-bearing changed relatively little, but ‘vagueness and inconsistency’ remained. What exactly were the ‘arma illicita’, for example, that clergy were frequently forbidden to carry? It was the first provincial council held by Carlo Borromeo, as archbishop of Milan, in 1565 that produced what Duggan considers ‘one of the most precise and thorough decrees on the whole issue ever promulgated in the history of Christianity’. Clergy were not to bear weapons of any sort except little knives designed for domestic use, unless they had to travel outside a city ‘in suspect places’. A bishop might grant permission in writing to carry arms for ‘probable and just cause’, but ‘crossbows, spears, arquebuses, daggers, and other weapons of this sort’ were entirely forbidden. (Which weapons bishops might permit to be carried under exceptional circumstances remains unclear.) A separate chapter deals with the especially ‘patchy’ legislation of the Church in England. Here, the ‘last and most significant prohibition’ was a canon issued by the papal legate Ottobono in 1268. Interpreted during the next two centuries as forbidding the bearing of ‘offensive’ weapons, and thus implicitly allowing self-defence, it came in modern times to be seen (incorrectly, Duggan contends) as a total prohibition of clerical arms-bearing. Two preliminary chapters contain examples of actual armed activity and of contemporary opinions on the subject spanning the centuries between late antiquity and the recent past. These chapters contain much interesting material, but they are too anecdotal, miscellaneous, unsystematic and uneven in coverage to complement effectively the more tightly controlled discussion of canon law that follows. They are far more concerned with warfare than with self-defence, while self-defence bulks rather larger than warfare in the chapters devoted to canon law. Throughout, a sharper analytical distinction between different types and levels of armed conflict would have been helpful.