Requested by the White House Office of Science and Technology, the National Research Council made a comprehensive study of the human health effects and public policy issues associated with the prospective increase in the use of diesel-powered light-duty vehicles in the US. The purpose of the study was to inform those government bodies, along with Congress, the automotive industry, and the American Public, about the current state of knowledge and understanding of diesel engine emissions and their control and to provide an authoritative and balanced examination of the risks and implications of the anticipated growth in the number of lightweight diesel vehicles. The Panel examined an approach to the selection of the best timing and level for regulating diesel engine emissions from passenger cars and small trucks.