Only about 15 percent of the known species in the United States have been studied in sufficient detail to determine whether or not they are imperiled. Any estimate of the total number of imperiled species in this country must therefore rely on extrapolations from this small number of comparatively well-studied species to a much larger number of poorly studied ones. We review the best available data on the status of plants, animals, and fungi in the US and conclude that the actual number of known species threatened with extinction is at least ten times greater than the number protected under the Endangered Species Act (ESA). The key to developing a more accurate picture of the extent of species endangerment is to obtain more data on the following groups (in decreasing order of priority): (1) invertebrate animals; (2) fungi; and (3) marine organisms. However, given the slow pace at which species are being protected under the ESA, and the rapid rate at which natural areas are being destroyed, a more urgent task is to develop and refine approaches to conservation that complement species-by-species protection, most notably the use of coarse filters.