How do international organizations (IOs) help states cooperate? This book will introduce a distinct information problem that hinders efforts to solve shared problems, and describes how IOs with unique secrecy functions can address them. We argue that states often possess unique information about other countries‘ violations of international rules. Without an institutional solution, states react to dilemmas of disclosing such details by keeping it private. As a result, commercially sensitive economic information and national intelligence often do not serve as inputs into questions of compliance. Yet IOs equipped with “confidentiality systems”—secrecy by another name—can analyze and act on such sensitive information, while preventing the harm that would come from wider release. I will present the theory chapter from this book project. The larger book assesses the theory in chapters on nuclear proliferation, international trade, tribunals for war criminals, and disputes about foreign direct investment.