This chapter surveys theories of regionalism and proposes a research agenda to study regionalism in a more inclusive and pluralistic fashion. We argue that much of the theorizing on regionalism is either implicitly or explicitly based on the European integration experience (EU-centrism) or is deductively derived from general International Relations theories with their tendency for Western-centrism. Thus, this chapter seeks to shift scholarly attention towards other, more critical approaches that we believe hold considerable merit in the study of regionalism. After surveying mainstream theoretical approaches to the study of regionalism, we highlight critical perspectives that have already engaged with regionalism or hold much potential in doing so. Decolonial, postcolonial perspectives, and their variants present a critical historical and political lens to theorizing beyond Europe. We conclude by suggesting that regionalism's current theoretical work could benefit from a broader engagement with critical scholarship in engaging alternative knowledge, historicizing scholarship and theorizing with regions as the basis.