Various aspects of the design and evaluation of hybrid/electric vehicles are considered with emphasis on the consequences of utilizing advanced electric driveline components such as AC motors/electronics and ultracapacitors. Special attention is given to series hybrid drivelines, because they benefit much more directly than parallel hybrid drive lines from the recent large improvements in the specific weight and volume of electric drive motors/electronics. The results of the present study indicate that series hybrid vehicles with an electric range of 90-100 km and good acceleration performance (0-88 km/h acceleration times of less than 12 seconds) can be designed with a powertrain weight and volume comparable to that of a parallel hybrid of the same performance. The driveline efficiencies of the series and parallel designs for both city and highway driving differ by less than 15 percentage ponts. The control of the series hybrid driveline is expected to be significantly simpler than that of the parallel hybrid system and in addition, meeting the California ULEV emission standards should be less difficult for the series hybrid design, because the start of its engine can be delayed until the catalyst is warm without affecting vehicle driveability.Work supported by the U.S. Department of Energy Assistant Secretary for Conservation and Renewable Energy (CE), under DOE Idaho Field Office, Contract DE-AC07-76ID01570.Simulation results for series hybrid vehicles on the FUDS and the Federal Highway cycles indicate that their fuel economy (miles per gallon) operating in the hybrid mode will be 25-50% greater than conventional ICE vehicles of comparable interior size. Hybrid/electric vehicles using ultracapacitors to load level the engine in the driveline showed even a greater potential improvement in fuel economy. Load leveled operation of the engine may make it less difficult to use high specific power engines, such as two-stroke and gas-turbine engines, in light duty vehicles having stringent emission control requirements.