The paper gives results of two workshops held by the U.S. EPA (7/19-20/90 and 8/12/90 in Washington, DC, and Sacramento, CA, respectively) to solicit research ideas from outside the Agency on research opportunities to improve the state of the art in estimating highway vehicle emissions. EPA's Joint Emission Inventory Oversight Group is initiating research projects with a goal of better quantifying air pollutant emissions in both current and future years. Highway vehicles have been found to be important contributors to organic, oxides of nitrogen, and carbon monoxide emissions as well as some hazardous air pollutants, despite the significant emission reductions that have already been achieved on a per-vehicle basis.