This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book addresses the relationship between the papacy and the crown. It discusses the relationship between the papacy and the church in Aragon-Catalonia. The book describes the coronation of Peter II at Rome and its aftermath. A coronation can reveal more of the shared intellectual inheritance and the divergence in political ideas of regnum and sacerdotium than almost any other event. The history of the Crown of Aragon in the period of its early rulers has received far less scholarly attention than it merits, not forgetting the magisterial studies of Salrach, Bisson and Ventura in particular. The book attempts to ask questions concerning the manner in which Innocent intervened in the affairs of the church and realm of Aragon-Catalonia, the results of his interventions and what, in the final analysis, were the pope's intentions.