In international regimes research, one of the most important questions is how effective regimes are in delivering what they were established and designed to achieve. Perhaps the most explicit and rigorous formula for measuring regime effectiveness is the so-called Oslo-Potsdam solution. This formula has recently been criticized by Oran Young, himself one of the founding fathers of regime analysis. The present article reviews and responds to his critique and provides several extensions of the Oslo-Potsdam solution. Our response may be summarized in three points. First, we recognize that difficult problems remain unsolved. Second, we argue that for some of the most profound problems there is no escape; we need to engage in counterfactual reasoning, and we need some notion of the “best” solution achievable (such as the “collective optimum”). Finally, we would welcome efforts to further develop and refine the Oslo-Potsdam formula as well as alternative approaches.