This chapter summarizes the major findings of the Handbook. The rise of regionalism exemplifies both a proliferation of regional (trade) cooperation, and a broadening and deepening of existing forms of regionalism. Regional organizations have taken on new tasks and moved towards regional integration, resulting in distinct combinations of regionalization and regionalism in various parts of the world. The authors offer tentative explanations for the emergence and the institutional design of regionalism, including diffusion and normative emulation among regional actors. They also explore the effects of regionalism. While the welfare-creating effects of regional trade regimes are unclear, the authors note the positive effects of regionalism on democracy, human rights, and peace and security, but acknowledge that the “dark sides of regionalism,” e.g. boosting authoritarian, rent-seeking regimes, have not received sufficient attention. They conclude with some methodological challenges of studying regions in a comparative way and with observations for future research.