Diesel engines in light and heavy-duty vehicles emit significant amounts of oxides of nitrogen (NOx), particulate matter, sulfur oxides, and unburned hydrocarbons. Emissions of these pollutants are strongly affected by the quality and composition of the diesel fuel used. This report reviews the published literature concerning fuel effects on diesel emissions, and examines the feasibility, costs, and cost-effectiveness of reducing diesel emissions by means of changes in diesel fuel composition. Two specific changes are considered: a drastic reduction in diesel fuel sulfur content; and the same sulfur reduction along with a moderate reduction in the aromatic hydrocarbon content. The effects of each change on engine durability, fuel economy, and refining costs are considered together with the emissions effects. The results indicate that the cost of producing low-sulfur diesel fuel would be more than offset by savings in reduced maintenance and greater engine life. Re-optimizing the refining process to produce low-sulfur fuel would also reduce the aromatic hydrocarbon content, improving ignition quality and reducing particulate emissions. A further reduction in aromatic content would be feasible, and would have superior cost-effectiveness to some other particulate control measures such as particulate traps.