Regional sub-state diplomacy has come of age. No longer limited to federal states in Europe, today sub-state entities across the world engage in international relations, and conduct a “foreign policy” parallel to, complementary to or sometimes in conflict with their central governmental counterparts. Since the late 1990s, the spectrum of diplomatic instruments and the strategies that accompany them have become more diverse and complex. Regional Sub-State Diplomacy Today offers detailed and recent data on the nature, width and complexity of regions engaging in international relations. It includes cases from all over the world. Next to comparative empirical studies, Regional Sub-State Diplomacy Today also offers original theoretical perspectives on the multi-faceted dimensions of regional sub-state diplomacy. It is ideal for both students and practitioners of sub-state diplomacy.