This book provides the only comprehensive analysis of concordats, the international treaties of the Apostolic See in Rome. Identifying the 167 treaties between the papacy and civil commonwealths from 1865 to 2022 at the intersection of canon, comparative, and international law, Royce indicates an overall relationship between the dominance or inferiority of Roman Catholic canon law within the contracting party and the respective ecclesiological or ideational norms of its concordat. Successive case chapters analyzing the concordats with fascist Europe, the German Länder, Latin American countries, France and Austria, the states of the Second Vatican Council, and Third World states illustrate that the norms of concordats with polities of long-standing, entrenched, continuous, or otherwise dominant Roman Catholic canon law concern the Church as an institution, whereas those with polities of new, precarious, inconstant, or otherwise inferior canon law status concern the Church as anadherent to values. This contractual law of the Apostolic See most closely aligns with the tenets of the English School of international theory. As a result, this book posits significant theoretical, legal, and empirical advances in existing knowledge of the international relations and law of the Catholic Church.