The transportation sector currently accounts for approximately two-thirds of all U.S. petroleum use and roughly one-fourth of total U.S. energy consumption. Substitution of petroleum-based transportation fuels (gasoline and diesel) by nonpetroleum-based fuels (alternative fuels such as electricity, ethanol, hydrogen, liquefied petroleum gas, methanol, and natural gas) could be a key means of reducing the vulnerability of the U.S. transportation sector to future disruptions of petroleum supply. This paper discusses transportation energy trends, the potential for alternative fuels in transportation, and research needs. Basic challenges facing each alternative fuel are examined.