The relationship between the physical and chemical properties and performance of gasolines and diesel fuel are discussed based on work reported in the literature. For gasolines, emphasis is placed on the effects of oxygenates when used as gasoline blending components. With the phase-out of leaded gasoline, and the eventual return of short gasoline supply, the use of oxygenated blending agents will increase in order to meet the octane demands for unleaded gasolines. Octane and volatility are the two most important quality aspects of gasolines. Oxygenates, particularly methanol and ethanol, have definite effects on these properties. The correlation between gasoline composition and its effect on other areas, e.g., emissions, water tolerance and elastomers, are also shown.For diesel fuel, emphasis is placed on the API Gravity, cetane, cloud and pour point, and stability. A short summary describing the properties and performance of alternate fuels, ethanol and vegetable oil for diesel engines (CI), is also included.Today's conventional automotive fuels can be broadly categorized as consisting of either gasoline, gasoline-oxygenate blends or diesel fuel. Alternate fuels, such as methane (CNG-compressed natural gas), propane (LPG-liquefied petroleum gas), and neat methanol and ethanol are also in use today, but only to a limited extent, primarily in fleet-type operations. Vehicle modifications are required in order to operate on alternate fuels. This paper will concern itself with the properties and performance characteristics of conventional fuels since these are expected to continue to be our major fuel sources, at least for the remainder of this century.