The operation of batteries in hybrid electric vehicles (HEVs) involves unusual constraints not seen in other applications. This paper reviews the specifications and operational requirements imposed on batteries due to the projected architectures for HEVs as defined by the DOE/PNGV Program. It also reviews the performance issues involved in battery HEV operation and surveys the strengths and weaknesses of the candidate electrochemical technologies. Finally, battery designs are recommended for the two major projected HEV applications, namely the so-called "fast-response" and "slow-response" systems identified in the DOE/PNGV Programme.