Given that few readers will approach the book with expertise in both England and Byzantium, this chapter provides essential background for the comparative study of ecclesiastical history in the two societies. It comprises three historical overviews covering: (1) the organisation of the two Churches in the twelfth century, including information on the number and nature of bishoprics, as well as the duties and rules governing the different ecclesiastical ranks; (2) the legal interactions between East and West up until the twelfth century, including their knowledge of different conciliar and patristic canons; (3) the development of the rules of clerical abstinence up to the twelfth century.