Professor Somerville deals here with the history of Latin Christianity at a crucial time - the century of the Gregorian reform movement and of the Investiture conflict between the papacy and the empire. The articles are concerned with the policies of the popes, as expressed in their letters and the canons of the councils they summoned, and with the impact on the life and laws of the Church. Conciliar history, indeed, forms the main focus of the volume, and the author's aim has been to subject the relevant texts and manuscripts to detailed scrutiny in order to determine their veracity and chronology. In so doing he also demolishes some of the pseudo-historical problems that have arisen from an uncritical reliance upon early printed editions. This investigation of the texts is of evident importance for the study of canon law, but it also shows how they can serve as valuable sources for the history of the Western Middle Ages, revealing much about life in the period, as well as about papal politics. Le professeur Sommerville traite ici de la chr ti nte latine au coeurs de la p riode cruciale que fut le si cle du movement de r forme gr gorien et du conflit d'investiture entre la papaut et l'empire. Ces tudes se pr occupent de la politique des papes, telle qu'on peut la voir exprim e au travers de leurs lettres et de canons issus des conciles qu'ils r unissaient. Elles s'int ressent aussi leur influence sur la vie et les lois d'Eglise. L'histoire conciliare forme, en effet, la plus grande part de ce receuil et l'auteur s'y propose de soumettre textes et manuscrits appropri s une tude d taill e, afin d'en d terminer la v racit et la chronologie. Ce faisant, il limine aussi un certain nombre de probl mes pseudo-historiques, subvenus en raison de la trop confiance accord e aux editions anciennes. Cette enqu ate men e sur les textes est, de toute vidence, d'une grande importance en ce qui concerne l' tude du droit