This chapter presents an operational version of regional security complex theory (RSCT). RSCT provides a conceptual frame that captures the emergent new structure of international security (1 + 4 + regions): hence our title Regions and Powers. As we have shown, RSCT has a historical dimension that enables current developments to be linked to both Cold War and pre-Cold War patterns in the international system. It contains a model of regional security that enables one to analyse, and up to a point anticipate and explain, developments within any region. RSCT provides a more nuanced view than strongly simplifying ideas such as unipolarity or centre–periphery. But it remains complementary with them, and provides considerable theoretical leverage of its own. In an anarchically structured international system of sufficient size and geographical complexity, RSCs will be an expected substructure, and one that has important mediating effects on how the global dynamics of great power polarity actually operate across the international system. This makes the theory interoperable with most mainstream realist, and much liberal-based, thinking about the international system. In another sense, the theory has constructivist roots, because the formation and operation of RSCs hinge on patterns of amity and enmity among the units in the system, which makes regional systems dependent on the actions and interpretations of actors, not just a mechanical reflection of the distribution of power. Wendt (1999: 257, 301), for example, makes the connection explicit, pointing out that his social theory can be applied to regional security complexes.