Two of CaliforniaÕs greatest environmental challenges are to meet national ambient air quality standards and to reduce greenhouse gas emissions that cause global warming. Although California has been struggling with air quality problems for more than four decades, concern over climate change is a relatively new phenomenon. Yet, the common ground between these two concerns is evidentÑboth air quality and climate change policies aim to reduce the harmful pollutants that threaten the publicÕs health and well-being. And one of the major culprits in both cases is the sameÑmotor vehicles, the leading source of both smog-forming and greenhouse gas emissions. This study examines options for reducing emissions from motor vehicles and evaluates each of the options in terms of its public health, climate change, and cost implications, including the uncertainty associated with each option. We examine battery-electric vehicles, fuel cell vehicles, the use of ethanol blends in flex-fuel vehicles, and reductions in vehicle miles traveled. We find that increasing the use of battery-electric vehicles provides the greatest public health benefit per unit of GHG emission reduction, followed closely by the use of fuel cell vehicles, and then by reductions in vehicle miles traveled. However, all of these options involve tradeoffs, and none ranks favorably along all dimensions. For example, battery-electric and fuel cell vehicles provide significant public health and climate change benefits, but both options involve high cost and uncertainty. Flex fuel vehicles consuming fuel blends containing ethanol derived from corn, on the other hand, have fairly low technological uncertainty, but do not provide any significant public health or climate change benefit. Looking ahead, California needs to design policies that will reduce emissions from the transportation sector at a reasonable cost, while achieving maximum benefits for both public health and the climate. Policymakers, industry leaders, and the public need to understand the tradeoffs among these goals and seek to reconcile them. For example, there is still considerable uncertainty surrounding battery-electric and fuel cell vehicles, which will depend on technological breakthroughs and broader market penetration to reduce cost and meet performance targets. And while biofuels may help reduce global warming, their benefits will be greatly reduced if future policies are not also designed to account for their impacts on land use and their potentially adverse effect on food prices, depending upon the material used in their production. In the concluding sections of this paper, we discuss CaliforniaÕs policy goals relating to air quality and climate change and the role of the transportation sector in meeting these goals. We also evaluate some of the policy options that California is likely to consider in terms of their climate benefits, public health impacts, and cost.