California's Zero Emission Vehicle (ZEV) program requires that starting in 2003, a proportion of the new vehicles delivered for sale in the state must produce no emissions. This study examines the costs and emission benefits of battery-powered electric vehicles, direct hydrogen fuel-cell vehicles, and other advanced technology vehicles, such gasoline hybrid electric vehicles, that manufacturers may use to satisfy program requirements. The authors find that the cost of battery-powered electric vehicles per ton of emissions reduced is very high and not likely to fall to reasonable levels any time soon, and that it is too early to tell whether direct hydrogen fuel-cell vehicles, which show much promise, are an economical way to improve California's air quality. The study also evaluates California's goal of reducing the emissions of the state's light-duty vehicle fleet to zero. It finds that federal air quality standards can be met without a zero emission fleet and that lower-cost alternatives for improving air quality appear available. The authors conclude that instead of requiring manufacturers to meet emission reduction targets with particular vehicle technologies, California should eliminate the requirement for zero emission vehicles, tighten emission standards on light-duty vehicles, and focus on setting emission performance standards.