On the world stage, the Holy See acts as both a religious and a political actor. As the head of over 1.2 billion Catholics, the pope is a widely recognized spiritual authority. Politically, the Holy See maintains diplomatic relations with other states and actively participates in international organizations such as the United Nations. A Living Tradition examines the normative sources and the dilemmas underpinning papal diplomacy. It does so in the context of four diverse case studies: the Vietnam War, John Paul II and Poland, the United Nations conferences in Cairo and Beijing, and the global campaign for debt relief. While Catholic Social Doctrine offers a principled basis for Holy See diplomacy, living out religious norms is more complicated than simply preaching them, especially in global politics. This process leads to political and ethical policy dilemmas as well as to changing patterns of conflict and cooperation with other international actors. By drawing upon unpublished archival documents from five countries, A Living Tradition offers a fresh and interdisciplinary view of both Catholic Social Doctrine and papal diplomacy that explores a key issue of the religious resurgence we are experiencing in the twenty-first century: how religious traditions function in global politics.