Pope Urban II's councils of Piacenza, Clermont, and Nimes feature prominently in every history of the First Crusade, but the major part of their business—the provision of authoritative solutions to the theological and ecclesiological issues that had been dividing the Church—is generally ignored. It was this that was of real significance to the participants, for whom the event for which the council of Piacenza is best known—the embassy bringing an appeal for assistance from the Byzantine emperor that provided a casus belli for the crusade—was a side issue. Robert Somerville, the leading historian of the Church councils of the late eleventh and early twelfth centuries, already has written the standard accounts of the Councils of Melfi of 1189 and of Clermont of 1095, the latter being a particularly difficult one to analyze and interpret. The heart of this outstanding book is an edition and translation of the canons of Piacenza, a truly significant council not only in the eyes of the pope, who treated it as though it were a general council, but also for the history of canon law, since almost the full text was incorporated into Gratian's Decretum. Chapters on the background to the council, the enactment, circulation, and survival of the canons and the work on them of earlier scholars precedes the edition of the canons. It is followed by a commentary on the canons and by a discussion of the councils that followed Piacenza, especially that in Rome of April 1099, which Somerville considers to be so important that a serious examination of it is an imperative.