A Special Report of the Institute's Diesel Working Group. Diesel engine emissions have the potential to cause adverse health effects, including cancer and other pulmonary and cardiovascular diseases. However, it is difficult to distinguish the potential health risks attributable to exposure to diesel exhaust from those attributable to other air pollutants. For over a decade, HEI has supported a broad-based research program to evaluate the health risks of diesel emissions, including investigations of carcinogenesis, modeling studies, and emissions characterization. The purpose of this Special Report is to examine what is known, not known, and still uncertain about the health risks of exposure to diesel emissions.