Risk assessments are often used by the federal government to estimate the risk the public may face from such things as exposure to a chemical or the potential failure of an engineered structure, and they underlie many regulatory decisions. Last January, the White House Office of Management and Budget (OMB) issued a draft bulletin for all federal agencies, which included a new definition of risk assessment and proposed standards aimed at improving federal risk assessments. This National Research Council report, written at the request of OMB, evaluates the draft bulletin and supports its overall goals of improving the quality of risk assessments. However, the report concludes that the draft bulletin is "fundamentally flawed" from a scientific and technical standpoint and should be withdrawn. Problems include an overly broad definition of risk assessment in conflict with long-established concepts and practices, and an overly narrow definition of adverse health effects--one that considers only clinically apparent effects to be adverse, ignoring other biological changes that could lead to health effects. The report also criticizes the draft bulletin for focusing mainly on human health risk assessments while neglecting assessments of technology and engineered structures.