Institutionalists commonly assume that the operation of regimes accounts for much of what happens in international society. Realists and neorealists, by contrast, typically regard institutions as epiphenomena that reflect deeper forces in international society and that can be expected to change when the deeper forces change. As is so often the case in debates of this nature, the truth no doubt lies somewhere between these polar perspectives. To identify the signal of the effects of institutions and especially to track variations in the strength of this signal, we need to find ways to draw clearcut inferences about the causal links between institutions and collective outcomes at the international level. Ideally, we should also devise an integrated index of regime effectiveness that would allow us to compare and contrast different regimes or the same regimes over time in terms of their effectiveness. This article offers a critical review of the leading efforts to develop useful inferences and indices with particular reference to international environmental regimes. It concludes that our efforts in this realm to date have yielded only modest—though hardly trivial—results. Yet we are far from exhausting the available analytic resources in this field, and there is much that can be done to improve inferences and indices in this important area of research in the future.