In the context of increasing fragmentation and functional differentiation in international governance, new environmental regionalism represents a recent trend involving initiatives that seek to territorialize environmental governance at the level of transboundary ecoregions, such as mountain ranges or river basins. This article examines the implications of this trend for sustainable development, which is defined here as a procedural norm for reconciling the tradeoffs between environmental, economic, and social dimensions of wellbeing. This article (1) traces arguments concerning the origins of functional differentiation to research on European state-making; (2) offers two complementary perspectives that generate insights into sustainable development at the transboundary level, one focusing on the intersection of multiple and overlapping functional spaces, and the other focusing on regionalization as the domestic manifestation of regional themes; and (3) illustrates the significance of these perspectives in the case of the European Alps. The article suggests that the Alps serve both as the bounded object of an international legally binding agreement asking its signatories to formalize sustainable development, and as the intersection of multiple overlapping functional spaces. It lends support to claims about the link between rescaling and functional differentiation, but demonstrates that a sympathetic critique of new environmental regionalism need not conclude that the phenomenon exacerbates the fragmentation of international governance.