The book reconstructs how the normative yardsticks that underpin evaluations of international organizations have changed since 1970. Based on in-depth case studies of normative change in five international organizations over a period of five decades, the authors argue that, these days, international organizations confront a longer and more heterogeneous list of normative expectations than in previous periods. Two changes are particularly noteworthy. First, international organizations need to demonstrate not only what they do for their member states, but also for the individuals in member states. Second, while international organizations continue to be evaluated in terms of what they achieve, they are increasingly also measured by how they operate. As the case studies reveal, the more pluralist patchwork of legitimacy principles today’s international organizations confront has multiple origins. It includes the politicization of expanding international authority, but also a range of other driving forces such as individual leadership or normative path dependence. Despite variation in the sources, however, the consequences of the normative shift are similar. Notably, a longer and more heterogenous list of normative expectations renders the legitimation of international organizations more complex. Strikingly, then, at a time when many feel international cooperation is needed more than ever, legitimating the forms in which such cooperation takes place has become most difficult. International organizations have come under pressure.