The EPA has been monitoring exhaust emission levels from in-use vehicles since 1971. The Emission Factors Program indicates that a potential problem exists with in-use vehicle emissions performance when these in-use vehicles are tested in the as-received state of tune-up and maintenance condition. Emission levels have been in excess of the Federal Emission standards. Other EPA Surveillance Programs have shown that in-use vehicles when maintained and tuned to manufacturers specifications are in substantial compliance with federal standards. This study employs currently available data sources in ascertaining the cause(s) for such differences in emission-related state-of-tune, and explores and evaluates potential technical solutions. Technological approaches to improving in-use performance of vehicles are divided into two classes: those involving technological changes to the vehicle; those involving technological changes in areas other than to the vehicle.