Abstract: Scholars and practitioners have long argued that easing access to information makes violations of international rules more visible and improves outcomes. This book introduces a different information problem -- disclosure dilemmas -- and argues that equipping IOs with secrecy can be critical to eliciting sensitive information and making cooperation work. It posits that states and firms often possess important information regarding compliance with international rules but are deterred from disclosing it by fears of revealing intelligence sources or helping commercial rivals. Confidentiality systems in international organizations can protect such sensitive details by allowing disclosure without wider dissemination. The book offers a unified, multi-method approach to understanding international cooperation and how institutions work, spanning economic and security domains from nuclear proliferation to trade to human rights. Typically treated separately in the field of international relations, this book bridges the divide and, in doing so, provide theoretical, empirical, normative, and practical contributions.