The majority of automobiles on the road today exceed the emission standards to which they were designed. This failure to achieve the design emission levels is primarily attributed to improper maintenance, tampering with emission controls, and misfueling catalyst cars with leaded gasoline. A method of reducing improper maintenance, tampering, and misfueling is periodic emission inspection and maintenance (I/M) programs.I/M's ability to deal with in-use emission problems is discussed and contrasted to the other mobile source emission control strategies. The results of EPA's evaluation of the Portland, Oregon, I/M program are presented. This study shows I/M to be extremely effective in reducing emission levels.A question remains, however, as to whether I/M will be a necessary emission control strategy for the generation of electronically controlled, 3-way catalyst vehicles that will be introduced nationwide in 1981. Recent data on California models and several 1981 prototype vehicles utilizing these control systems show that failures of these new systems, or failure to replace the vehicle's oxygen sensor, can result in extremely high emission levels. The potential for the high emission levels to occur in-use is discussed, and preliminary data on the various short tests' ability to detect high emitting vehicles is presented.