The number of regional organizations and regional trade agreements has risen sharply since the 1990s. In its wake, comparative research on regionalism has seen a revival. An important strand of this literature asks about the drivers of these developments, but has to date largely neglected a puzzling phenomenon: the similarities between regional organizations in their institutional design and the chosen methods of integration. Existing perspectives analyze different cases of regionalism primarily as phenomena that develop independently of each other, and whose genesis and design are determined either endogenously by domestic regional dynamics or exogenously by powerful hegemons. Against this background, this article argues for an extension of existing analytical perspectives and sketches a diffusion-oriented research agenda that conceives of regional organizations as interdependent phenomena.