This chapter focuses on the diffusion of regionalism, regional organizations (ROs), and regional governance. The chapter conceptualizes diffusion in terms of initial stimuli, objects of diffusion, mechanisms, and outcomes. The author reviews the existing literature focusing on the diffusion of regionalism and regional orders, of institutional designs for ROs, and of regional governance pertaining to specific policy areas. The diffusion of regionalism, institutional designs, and regional governance is primarily determined by indirect mechanisms of emulation rather than by direct—“sender-driven”—influence mechanisms. As far as emulation is concerned, we observe few examples of mimicry leading to institutional isomorphism. Competition, lesson drawing, and normative emulation are the most important diffusion mechanisms by which regionalism spreads. As to diffusion outcomes, we rarely observe full-scale adoption or convergence around specific models of regional cooperation and integration. The most likely outcome of diffusion with regard to regional cooperation and integration is adaptation and transformation.