The role of lead antiknocks in modern gasolines is discussed in terms of engine-fuel relationships. Exhaust emission characteristics of leaded and unleaded gasolines are compared in terms of both gaseous and particulate constituents. The effect of removing lead from gasoline on engine cleanliness, exhaust valve seat recession, octane requirements, and octane requirement increase is assessed. Extensive use is made of published information and some new information is presented.It is apparent that the use of lead antiknocks in gasoline produces effects on engine performance and exhaust emission characteristics which are both positive and negative in nature. Much more information is needed, particularly in terms of future vehicles equipped with advanced control systems, to determine the optimum fuel composition. In view of this situation, the best course of action appears to be one which provides for maximum flexibility, both in systems to meet future emissions and in the use of lead antiknocks.