As used in public health and environmental regulations, risk assessment is the systematic, scientific description of potential harmful effects of exposures to hazardous substances or situations. It is a complex but valuable set of tools for federal regulatory agencies to identify issues of potential concern, select regulatory options, and estimate the range of a forthcoming regulation's benefits. However, given the significant yet controversial nature of risk assessments, it is important that policymakers understand how they are conducted, the extent to which risk estimates produced by different agencies and programs are comparable, and the reasons for differences in agencies' risk assessment approaches and results. GAO studied the human health and safety risk assessment procedures of the Environmental Protection Agency, the Food and Drug Administration, the Occupational Safety and Health Administration, and the Department of Transportation's Research and Special Programs Administration. This report describes (1) the agencies' chemical risk assessment activities, (2) the agencies primary procedures for conducting risk assessments, (3) major assumptions or methodological choices in their risk assessment procedures, and (4) the agencies' procedures or policies for characterizing the results of risk assessments.