In the United States, as in other industrialized nations, regulatory decisions to protect the environment and public health depend heavily on scientific information. Yet the process of decision-making places unusual strains on science. Knowledge claims are deconstructed during the rule-making process, exposing areas of weakness or uncertainty and threatening the cognitive authority of science. At the same time, the legitimacy of the final regulatory decision depends upon the regulator's ability to reconstruct a plausible scientific rationale for the proposed action. The processes of deconstructing and reconstructing knowledge claims give rise to competition among scientists, public officials and political interest groups, all of whom have a stake in determining how policy-relevant science should be interpreted and by whom. All of these actors use boundary-defining language in order to distinguish between science and policy, and to allocate the right to interpret science in ways that further their own interests. This paper explores the contours of such boundary disputes in the context of controversies over carcinogen regulation. It focuses on the contested definitions and strategic implications of three groups of concepts: trans-science or science policy, risk assessment and risk management, and peer review.