The characterization and operation of engines for hybrid-electric vehicles are reviewed with special attention being given to on-off engine operation and its effect on vehicle fuel economy and emissions. Rotary, two stroke, and gas turbine as well as four stroke gasoline and diesel engines are considered for hybrid applications. Various engine operating strategies are evaluated for range-extender and engine-electric series hybrid vehicles based on simulation results. The need for on-off engine operation to attain good fuel economy and low emissions in city driving is identified for series hybrids designed for unlimited range at highway speeds. Previous experience with the GE/JPL HTV-1 parallel hybrid vehicle in 1980-82 is reviewed as that vehicle successfully utilized on-off engine operation.