Biomonitoring has chronicled hundreds of synthetic chemicals in human bodies. With the proliferation of biomonitoring studies from diverse stakeholders comes the need to better understand the public health consequences of synthetic chemical exposures. Fundamental disagreements among scientific experts as to the nature and purpose of biomonitoring data guide our investigation in this paper. We examine interpretations of biomonitoring evidence through interviews with 42 expert scientists from industry, environmental health and justice movement organizations (EHJM), academia, and regulatory agencies and through participant observation in scientific meetings where biomonitoring evidence is under debate. Both social movements and industry stakeholders frame the meaning of scientific data in ways that advance their own interests. EHJM scientists argue that biomonitoring data demonstrates involuntary Btoxic trespass^and underscores a policy failure that allows for the pervasive use of untested chemicals. Industry scientists seek to subsume biomonitoring data under existing regulatory risk assessment paradigms. Our analysis reveals one area of convergence (validity of Centers for Disease Control surveillance data) and seven areas of contestation regarding the scientific, public health, and policy implications of biomonitoring evidence, among regulatory, industry, and EHJM scientists including: chemical presence in bodies, biological mechanisms of health impact, use of biomonitoring equivalents, limits of targeted biomonitoring, limits of detection, policy influence of advocacy biomonitoring, and relevance of biomonitoring to motivate policy change. These areas of scientific contestation provide insight into the persistent challenges of regulating chemicals even in the midst of mounting evidence of widespread exposure to multiple compounds with implications for human health.