While a number of different theoretical models have been advanced to explain why states implement—or, indeed, do not implement—multilateral environmental agreements (MEAs), very little empirical work has been undertaken to validate their predictions. With a view to narrowing this gap, the present article adopts a large-N, econometric approach to test the explanatory power of four distinct models of compliance—domestic adjustment, reputational, constructivist and managerial—in the context of European Union (EU) environmental policy. Using data on the number of ofªcial infringements received by 15 member states for non-implementation of environmental directives over the period 1979–2000, we ªnd that all four models make a statistically signiªcant contribution to explaining spatio-temporal differences in legal implementation. Thus, our results suggest that the implementation of MEAs is shaped by a combination of rational calculations of domestic compliance costs and reputational damage, domestically institutionalized normative obligations, and legal and political constraints. We conclude by suggesting a greater need for multi-causal theoretical models of supranational legal compliance.