With the advent of the fleet average fuel economy standards for passenger cars and light trucks, the international passenger car and truck manufacturers have sought cost effective means of improving fuel economy. The dieselization of the passenger car and light truck fleets is a singularly important way to improve fuel economy. With the many unknowns concerning unregulated diesel emissions and their potential impact on human health, the responsible government agencies, NHTSA, DOE, and EPA, have taken a cautious attitude regarding a major shift into diesel engines until more is learned. All three agencies have been actively studying the problems of diesel emission measurement, control, dispersion, and chemical transformation and medical effects. This workshop, which had extensive participation from industry, universities, government agencies, and concerned citizens, provided an opportunity to exchange information among experts, policy makers, and interested parties in the areas of: (1) emissions characterization and measurement; (2) control technology; (3) factors affecting human exposure; and (4) health effects. As the proceedings show, there is much to be learned about unregulated diesel emissions.