The last few decades have been marked by the emergence of well over 100 environmental protection regimes in the international arena. While we have a reasonably good understanding of the conditions under which international agreements (IEAs) arise and international compliance with these agreements, we know comparatively little about their effectiveness at improving environmental conditions. Progress in this area requires advances in both theory and method. On the theoretical front, most theories purporting to explain the effectiveness of IEAs are either so complex as to generate over determined models (e.g., Miles et al. 2002) or too narrow in that they consider only �rational choice� explanations for institutional consequences (e.g. Barrett 2003). On the empirical front, most evaluations of IEA effectiveness have been case studies (either qualitative or quantitative) that have contributed little to generalizable knowledge about the design of effective IEAs. In our paper, we pursue three goals. First, we develop a relatively parsimonious theoretical framework that predicts the effectiveness of IEAs as a function of the institutional characteristics of the agreement, the characteristics of the environmental problem, the characteristics of the policy tools employed, and international connections reflected in the regime. Second, after developing a coding instrument operationalizing this framework, we apply this instrument to measure the predicted effectiveness of more than a score of European IEAs. Finally, we canvass the extant literature on the effectiveness of IEAs and compare the conclusions from this research with the predictions derived from our theoretical framework.