The analysis of foreign policy and diplomatic practices is undergoing a revival within the field of international relations. This twenty-third book by Professor BUGEME CHIRABA Jérôme explains how this renewal involves, on the one hand, the theory of international relations to privilege rationalist and systemic explanations (balance of power, hegemonic transitions, etc.) to the detriment of foreign policy making. On the other hand, transnationalist approaches contribute, to a certain extent, to relegating the State as an actor in international relations to the background and to overshadowing the study of its external action. The Holy See maintains diplomatic relations with nearly two hundred states. Although it has no military or economic resources, its diplomacy plays a significant role. Its guiding principles are, in addition to the defense of Catholics, peace, justice and the promotion of the common good.