In the United States, cost-benefit analysis (CBA) is in the ascendancy. For over twenty years, American presidents have required agencies to perform CBA for major regulations; indeed, they have told agencies to regulate only if the benefits of regulation justify its costs. Congress has also shown considerable interest in CBA, most prominently in the Safe Drinking Water Act, which asks agencies to produce quantitative assessments of both costs and benefits. For their part, federal courts have adopted a series of principles that promote CBA, saying that if Congress has not been clear, agencies may consider costs, take account of the substitute risks introduced by regulation, and exempt trivial risks from governmental control. In its enthusiasm for cost-benefit analysis, the United States provides a sharp contrast to Europe, which has shown intense interest in a quite different organizing principle for environmental protection: the Precautionary Principle. According to the Precautionary Principle, regulation is required even in the face of scientific uncertainty—even if it is not yet clear that environmental risks are serious. A central point of the Precautionary Principle is to recognize the limitations of existing knowledge and to protect against harm that cannot yet be established as such. Cost-benefit analysis and the Precautionary Principle can lead in