This report assesses the potential of substitutes for gasoline to reduce emissions of carbon dioxide and other greenhouse gases (GHGs) by automobiles and light-duty trucks. The report also assesses the cost-effectiveness of replacing gasoline with varied fuels as a strategy for reducing GHG emissions, and concludes that promoting alternative fuels would be a costly strategy for reducing emissions. The study also briefly surveys other concerns that are likely to arise in making a transition from near-exclusive reliance on gasoline to widespread production and use of any alternative fuel. These concerns include potential health and safety consequences, developing the infrastructure required to support commercial-scale production and distribution of gasoline substitutes, and producing adequate supplies of feedstocks required to refine certain fuels.