This paper discusses cross-border regionalism within the supranational contexts of the European Union and the developing North American Free Trade Area. Focusing on planning and regional development issues, cross-border regionalism as a new form of governance is investigated, based on a comparison of supranational integration logics, co-operation frameworks and instruments, and co-operation agendas and strategies. In both contexts, discrepancies between programmatic objectives and co-operation results are striking, eliciting questions as to the greater potential significance of cross-border regionalism. The paper argues that theories of political regulation and constructivist perspectives might help relate cross-border regionalism to broader economic, political and cultural variables, explaining how regionalist agendas and strategies emerge, and how they are a response to local interests and aspirations and to external economic and political pressures.