The U.S. Environmental Protection Agency has recently reduced the permissible concentration of lead in gasoline from 1.1 to 0.1 gram per gallon, and has proposed to eliminate lead entirely by 1988. In addition to its octane-enhancing properties, lead in gasoline protects exhaust valve seats in older engines from undue wear (“valve-seat recession), and it and its scavengers have numerous other positive and negative effects. These include changes in octane requirements, hydrocarbon emissions, engine rusting, corrosive wear, oil thickening and degradation, spark-plug fouling, exhaust-valve burning, and exhaust system corrosion. This paper reviews the literature on the harmful and beneficial effects of lead and lead scavengers on engines, and examines some of the substantial body of operating experience that has been accumulated with unleaded gasoline in older engines. Based on this experience, it does not appear that valve-seat recession will be a major problem, even if all lead is eliminated from gasoline. Furthermore, the switch to unleaded gasoline should provide significant benefits in the form of reduced maintenance costs and increased engine life.