Environmental concerns have been moving up on the agendas of policy makers in industrialized states, mostly involving debates about the stringency of environmental regulation. There are, however, two sides to environmental regulation that deserve attention. On the one side, there are factors that affect the definition of environmental law. On the other side, laws cannot clean up the environment unless they are properly monitored and enforced by governments and their responsible agencies. Comparatively scant attention has been paid by political scientists to this latter aspect. This article addresses the issue of environmental monitoring and enforcement and reviews the empirical research on European states. While there is some promising research on individual states, cumulative knowledge is hampered by the limited number of comparative empirical assessments. The main reason for this deficit, however, can be seen in difficulties regarding measurement and data availability. Inspired by the reviewed literature, the article suggests a measurement that takes into account changes in the legal competences of the agencies responsible, which might be helpful in overcoming the impediment to comparative analysis. Yet improving our insights by means of comparison is only one of several points that constitute the research agenda with respect to the study of environmental monitoring and enforcement in Europe. Thus, another contribution of this article is the identification of promising avenues for future research. Copyright © 2012 John Wiley & Sons, Ltd and ERP Environment