International organizations (IOs) dispatch fact-finding missions to establish epistemic authority by objectively and impartially assessing contested facts. Despite this technocratic promise, they are often controversial and sometimes even fuel international disputes that challenge the epistemic authority of the dispatching organizations. Although the twenty-first century has witnessed a proliferation of United Nations (UN) commissions of inquiry, they have received surprisingly little attention in international relations (IR) scholarship. How can we explain this trend and the successes and failures of fact-finding missions, which sometimes even backfire on the IO authority? Drawing on IR theories of delegation, epistemic authority, and IO field operations as well as public international law scholarship on commissions of inquiry, this article develops an analytical framework for studying the delegation, implementation, and dissemination of fact-finding missions. It theorizes how and under what conditions international fact-finding missions close or widen credibility gaps and thus help to establish, maintain, or weaken the epistemic authority of IOs. The article illustrates this framework with a case study of the Ad Hoc Working Group on the Human Rights Situation in Chile, sent by the UN Commission on Human Rights in 1974 to investigate allegations of human rights violations and torture. The conclusion outlines a comparative research agenda on international fact-finding missions for IR that contributes to the study of knowledge production in IOs and the enforcement of international norms.