This paper has been reviewed by the Office of Health and Environmental Assessment, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. In 1983, the U.S. National Academy of Sciences (U.S. NAS) proposed a framework for the processes of risk assessment and risk management in government agencies (U.S. NAS, 1983). Using the U.S. NAS scheme as an organizing principle, the U.S. Environmental Protection Agency (U.S. EPA) published guidelines pertaining to risk assessment in five areas: estimating exposures, chemical mixtures, mutagenicity, suspect developmental toxicity and carcinogenicity. These guidelines were developed to promote high technical quality and consistent practice of risk assessment Agencywide. This paper will discuss the historical development of the guidelines and their role in the work performed by the Agency. Each of the five (5) guidelines is outlined and anticipated revisions discussed. Related assessment activities and new subject areas are also presented.