This book owes its appearance to three main causes. The desire to work out the relations of the English Church with the Papacy in the eleventh and twelfth centuries had long lain at the back of my mind, but it was only when the Council of Trinity College did me the honour of inviting me to deliver the Birkbeck Lectures during the two following academic years that I received the necessary incentive and opportunity. Then, when I took the subject in hand, it soon became evident that everything depended on determining what was the law in the English Church in the eleventh and twelfth centuries, and that only an extensive search to discover what collections of ecclesiastical law were then in existence in England could provide a solution of that problem. To make such a search among all the available libraries in England necessitated a considerable amount of leisure and lengthy absences from Cambridge, and I am indebted to my University and my College for the year's leave of absence from my ordinary duties without which I could not have properly completed the lectures, still less have prepared them without delay for publication. Thirdly, the Syndics of the University Press generously offered to undertake the publication, and I am much indebted to the invariable kindness and expert assistance of their staff.

Substantially this book reproduces the twelve lectures which I delivered in the two academic years, 1929–31.