THIS paper seeks to explain cross-national variations in patterns of firm-level adoption of two supranational environmental management system (ems) standards: the European Union’s Eco-Management and Audit Scheme (emas) and the International Standards Organization’s iso 14001. ems standards encourage firms to voluntarily adopt policies that go beyond the requirements of extant law and promote continuous improvement in firms’ environmental performance. It is doubtful that political scientists twenty years ago would have paid much attention to the advent of such voluntary schemes, and they almost certainly would not have been interested in examining firms’ responses to them. Yet today ems standards can be seen as a part of broader trends that are fundamentally changing the way business and certain policy areas are regulated. Perhaps the two most prominent characteristics of this new form of governance are the increase in the amount of regulation being formulated within supranational forums (usually in the form of issue-specific regimes) and the greater reliance on private actors for implementing these supranational standards. While there has been a great deal of research on the formation and structure of supranational regulatory regimes, the implementation of these regimes by national governments and their effects on private actors 368remain underresearched. 1 This article addresses these gaps by examining firms’ responses to emas and iso 14001 in the United States, the United Kingdom, and Germany and then offering an institutional explanation for why this response has varied so widely. It concludes that the characteristics of both domestic (business-government relations) and supranational institutions (the nature of the policy regime) need to be taken into account to fully explain the cross-national variations in firm-level responses.