In this ambitious two-volume open access collection of essays, a diverse group of scholars worldwide reflects on the past, present and future of the (comparative) regionalism research program. Volume I provides building blocks for regionalism studies in the coming years by indicating what a pluralistic theoretical and methodological toolbox to study regions could look like, how disciplinary boundaries can be bridged, and the relevance of comparative regionalism research. In consecutive sections, the chapter authors present their answers to the following guiding questions: What are the global origins of regionalism thinking? How relevant is mainstream theorizing (still)? And, finally, how could/should global regionalism look like? This collection of essays celebrates and highlights the research and scholarly undertakings of the United Nations University Institute on Comparative Regional Integration Studies (UNU-CRIS) on its 25th anniversary.