JOHN C. BAILAR is a professor in the Department of Epidemiology and Biostatistics at McGill University. His primary professional interests focus on uncertainty in scientific inference and risk assessment. In 1990 he was awarded a MacArthur fellowship.JACK NEEDLEMAN is currently a fellow at the Malcolm Wiener Center for Social Policy, John F. Kennedy School of Government, Harvard University, He previously was vice president and co-director of the public policy practice at Lewin and Associates, a Washington, D.C. health policy and consulting firm.BARBARA L. BERNEY, a Pew health policy fellow, is a project director in the Department of Environmental Health, School of Public Health, Boston University. Her principal professional interests include community strategies for abating environmental health hazards and national health reform.J. MICHAEL McGINNIS is assistant surgeon general and director of the Office of Disease Prevention and Health Promotion in the U.S. Department of Health and Human Services. He also served as chair of the Departmental Task Force on Health Risk Assessment, which produced the report Determining Risks to Health: Federal Policy and Practice (Auburn House, 1986).