Informal institutions, like soft law and informal IOs, have been increasingly utilized as instruments of international cooperation. Recent studies seeking to account for this development have generally focused on explaining decisions about the legal design of institutions at the "constitutional moment" when a new agreement or organization is first created. However, these accounts only reveal part of the story. In fact, the legal design of institutions often changes over time. After they have been created, complex processes of formalization and informalization have often occurred. This project aims, first, to conceptualize and explain these processes. It maps the unique ways in which they have unfolded across issue areas, advances a theory to account for these patterns, and tests this theory using both quantitative and qualitative tools. Second, the project aims to assess how formalization and informalization processes affect the performance of international institutions. When do they help an organization to achieve its objectives, and when do they undermine this goal?  Finally, the project reflects on the policy implications of the developments it focuses on, aiming to shed light on the “levers” that policymakers can use to improve the performance of global institutions in the future.