To accurately assess the relative effectiveness of international environmental agreements requires that we pay greater attention to how problem structures influence both institutional design and the outcomes we use to evaluate institutional effects. Analyzing multiple agreements allows us to move beyond claims that an agreement was influential to claims regarding which variables explain that influence, to examine how institutional influence depends on other factors and to evaluate an agreement's effectiveness relative to other agreements and non-institutional influences. Accounting for problem structure is crucial to such endeavors because problem structure variables may be alternatives to or interact with institutional variables and because institutional design is endogenous to the problem structure-outcome relationship. The shortcomings related to incorporating problem structure in extant effectiveness research can be overcome through four strategies: carefully describing analytically useful variation in problem structure, selecting cases to limit variation in problem structure, evaluating problem structure variables and their influence on design and behavior, and evaluating effectiveness in terms appropriate to the problem structure.