The main objective of this article is to develop a better understanding of the structure of transnational regulatory scientific institutions (RSIs). We will argue that the hybrid political-legal-epistemic nature of RSIs creates a continual tension between their hierarchical and policy-driven structure and the paradigms of objectivity, parallelism and non-centralism that characterize science. The article examines the way in which RSIs cope with the challenge of maintaining their epistemic/political authority against the tensions generated by their hybrid structure. The article focuses on three institutions: the Intergovernmental Panel on Climate Change (IPCC), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the International Competition Network (ICN), and examines how this challenge manifests itself in the context of these three bodies. The article links the discussion of hybrid authority with the problem of scientific uncertainty. It concludes with a discussion of the optimal design of RSIs.