The main focus of this study was to assess the feasibility of reducing exhaust emissions through a program of mandatory inspection and maintenance. The study was conducted using the Economic Effectiveness Computer model program which is capable of predicting the emissions changes and costs associated with a wide range of inspection and maintenance procures. To support the model, data have been obtained experimentally which define the general maintenance state of large vehicle populations, the sensitivity of exhaust emissions to engine malfunctions, and the rate with which engine parameter deteriorate. The primary purpose of this report is to provide general guidance for those states contemplating inspection/maintenance programs as part of their implementing strategy for meeting the new federal air quality standards.