The adequacy of the science supporting implementation of the Endangered Species Act (ESA) is receiving increased congressional attention. While some critics accuse agencies responsible for implementing the ESA of using "junk science," others counter that decisions that should rest on science are instead being dictated by political concerns. Under the ESA, certain species of plants and animals (both vertebrate and invertebrate) are listed as either endangered or threatened according to assessments of the risk of their extinction. Once a species is listed, powerful legal tools are available to protect the species and its habitat. Efforts to list, protect, and recover threatened or endangered species under the ESA can be controversial. Some of this controversy stems from the substantive provisions of this law, which can affect the use of both federal and nonfederal lands. The scientific underpinnings of decisions under the ESA are especially important, given their importance for species and their possible impacts on land use and development. The Fish and Wildlife Service in the Department of the Interior and the National Marine Fisheries Service in the Department of Commerce administer the ESA, and each agency has policies and requirements to ensure the integrity and objectivity of the science that ...