How can we explain the emergence and institutional design of regionalism across the globe? 1 There are basically two stylized explanations for regional cooperation and integration (see Chapter 1 by Börzel and Risse, this volume, on these terms). The dominant approaches to regionalism argue that regional cooperation and integration result from independent decision-making within one region or part of the world (Chapter 3 by Börzel, this volume). Accordingly, regionalism and regional institutions are particular answers to functional problems, whether endogenous to a region (e.g. economic interdependence) or exogenous (e.g. globalization). A second account emphasizes interdependencies between regions or sub-regions. Regional organizations (ROs) do not exist in isolation from each other, but models of regional cooperation and integration spread across the globe (Jetschke and Lenz, 2013). The focus of this work is on the diffusion of institutional models and policies. The most far-reaching diffusion account stems from sociological institutionalism and claims that there are global scripts of what constitute legitimate institutions and that these scripts are emulated across the globe (Meyer, 1987; Powell and DiMaggio, 1991; Boli and Thomas, 1998). Regionalism might be one of these scripts. This chapter focuses on this second account, on the diffusion of regionalism, ROs, and regional governance pertaining to particular issue-areas. It should be stated at the outset that the two accounts are not mutually exclusive (Chapter 27 by Börzel and Risse, this volume). Functional explanations for regional cooperation and integration can be combined with diffusion accounts focusing on institutional design. In the end, it is a question of emphasis and of the specific research question one wishes to answer, whether one focuses on independent or interdependent decision-making. This chapter begins by conceptualizing diffusion in terms of initial stimuli, objects of diffusion, mechanisms, and outcomes. I then review the existing literature focusing, first, on the diffusion of regionalism and regional orders per se, second, of institutional