In international law, the exercise of discretion is subject to pre-established constraining criteria. These are typically not explicit in the norms that confer the discretion. This explains the importance of the scholarly study of constraining criteria as well as some of its methodological difficulties. Study is further complicated by the different functions that constraining criteria serve. As this chapter argues, constraining criteria reflect, each in its own way, two fundamentally different conceptions of an international legal system. In the one case, the international legal system is seen as a series of norms that serve the common purposes of the states and international organizations that create them. In the other, it is conceived as a system with certain inherent moral or technical features, which are partly beyond any lawmaker’s control. The chapter directs attention to a few judicial decisions that help to illustrate precisely this point.