Diesel engines in light and heavy-duty vehicles emit significant amounts of oxides of nitrogen (NOx), particulate matter, sulfur oxides, and unburned hydrocarbons. Emissions levels for these pollutants are strongly affected by the quality and composition of the diesel fuel used. The study briefly reviews the available data and examines the feasibility, costs, and cost-effectiveness of reducing diesel emissions by mandatory changes in diesel fuel composition. Two types of changes are considered: (1) a drastic reduction in diesel fuel sulfur content, alone or in combination with a moderate reduction in the fraction of aromatic hydrocarbons in the fuel and (2) effects on engine durability, fuel economy, and refining costs. The work here is intended as a preliminary evaluation of the feasibility and cost-effectiveness of diesel emissions control through fuel modifications.