The California Steam Bus Project demonstrated the potential of low-emission, quiet external combustion engines in public transit service. Three contractors supplied and installed steam powerplants in urban buses, replacing the original diesel engines. Exhaust emissions were found to be considerably lower than the 1975 California requirements for heavy-duty vehicles. Substantial reductions in sound levels were measured in one of the buses. Powerplant weights can be lower than present diesel engines. Road performance was similar to that with diesel engines, but very high fuel consumption was experienced with these nonoptimized demonstration vehicles. Prospects for future improvements are given in this paper, including the outlook for large reductions in fuel consumption and exhaust emissions.