The principle of environmental policy integration (EPI) attracts great scholarly interest as well as widespread political backing. Political support is particularly strong in the European Union, where it enjoys a prominent quasi-constitutional status. However, the practical fulfilment of EPI appears to lag well behind these aspirations, although the evidence base of this widely held view remains rather fragmented. This article aims to review the 'state of the art' in EPI research and practice from the perspective of its conceptual meaning, processes of implementation and outcomes 'on the ground'. It finds that the political commitment to EPI is indeed widespread, especially in industrialized states, but that deep disagreement surrounds its actual application. In terms of everyday practices, 'policy integration' is complex and contingent, and there are few 'best practices' that can be easily shared between jurisdictions. Finally, knowledge about policy outcomes is very sparse indeed, and policy-making systems seem very ill prepared to address this lacuna.