This paper provides an overview of the effects of blending ethanol with gasoline for use in spark ignition engines. The overview is written from the perspective of considering a future ethanol-gasoline blend for use in vehicles that have been designed to accommodate such a fuel. Therefore discussion of the effects of ethanol-gasoline blends on older legacy vehicles is not included.As background, highlights of future emissions regulations are discussed. The effects on fuel properties of blending ethanol and gasoline are described. The substantial increase in knock resistance and full load performance associated with the addition of ethanol to gasoline is illustrated with example data. Aspects of fuel efficiency enabled by increased ethanol content are reviewed, including downsizing and downspeeding opportunities, increased compression ratio, fundamental effects associated with ethanol combustion, and reduced enrichment requirement at high speed/high load conditions. The effects of ethanol content on emissions are also reviewed, including NMOG/CO/NOX, particulate matter, toxic compounds, and off-cycle and evaporative emissions.Considering the engine and vehicle-related factors reviewed in this paper, a mid-level ethanol-gasoline blend (greater than E20 and less than E40) appears to be attractive as a future fuel. To provide high knock resistance, this fuel should be formulated using a blendstock that retains the octane of the current blendstock used for regular-grade E10 gasoline. Further work is needed to recommend a specific ethanol blend level, including analysis of fuel efficiency and CO₂ benefits for representative powertrain/vehicle applications, and of fuel production and supply considerations.