Ever since the passage by Congress of the great wave of late twentieth century federal environmental statutes, two competing normative principles have shaped the debate over how federal regulatory agencies should implement the statutory command to protect the environment. On the precautionary principle, regulation of an environmental risk is justified whenever there is some credible scientific evidence of a potentially “significant” risk of irreversible harm to human health or the natural environment. On the competing principle, that of economically efficient regulation, risks to health or the environment should be regulated only if the benefits of a particular regulatory intervention – reducing such significant risks – outweigh its costs – which often include the creation of new, equally significant and equally irreversible risks.