The National Research Council was asked to do a study on assessing the principal methods that federal and state agencies are using or developing to rank hazardous waste sites for remediation priority. The committee was asked to consider the intended technical and policy purposes and actual uses of the methods in the ranking decision process; their effectiveness in achieving those purposes; the types and levels of uncertainty of the input data and the methods' resulting limitations; the methods' assumption; the appropriateness of the assumptions for the methods' intended purposes; the sources, magnitude, and treatment of significant uncertainties in each method; the sensitivity of the resulting score to the methods' computation process; and the methods' flexibility for follow-up evaluation of site assessments or for comparative analyses of the costs and effectiveness of remediation techniques. Also the committee was asked to identify the information and research needed to establish standards of performance and consistency for nationally applicable hazardous-waste site-ranking methods.