International agreements on issues related to human health and the environment often enlist notions of scientific principles or "science-based" decision making in order to constrain the realm of permissible causal argument within the an area of international law. As a result, parties to these agreements often wage conflicts over what is or is not "scientific," and what kinds of decisions are or are not "based on" science - with important ramifications for the sovereign regulatory rights of the parties. This dissertation explores this process of contesting and constituting epistemic authority in international health and environmental law, and makes a modest attempt to suggest pathways to constructing more broadly legitimate international practices for validating knowledge claims for taking collective international action. It does this through a detailed, mixed-method exploration of the practices employed by a number of critical international institutions to structure who is empowered to know, and to govern, in this relatively nascent international regulatory sphere. Through a mixture of negotiation observation, participant interviews, document review, and case law analysis, this dissertation tracks threads of cognitive and legal authority within and between science advisory bodies, domestic and international regulatory bodies, and domestic and international courts. By engaging with actual, as opposed to idealized sources of cognitive and legal authority in international affairs, it seeks to both illuminate the complexities of the relationship between science, sovereignty and the rule of law in global regulation, and to point the way to more broadly accepted practices of knowledge-making and law-making in global regulation. I have approached this issue in three different ways: an in-depth analysis of the techniques used by two different international adjudicative bodies for evaluating scientific claims; a comparative examination of the expertise-related administrative law of the United States and European Community with an eye to the emergence of international norms; and a detailed examination of the birth of a global agency and the rise of its authoritative discourse of risk analysis. All three of these studies take interdisciplinary approaches to addressing these issues, supplementing legal analysis of key treaties and cases with important insights and analytical techniques from the field of science and technology studies (STS). Each chapter addresses distinct but interrelated issues, and makes a separate contribution to our understanding of the relationship between epistemic and regulatory authority in global governance.