The increase in domestic supplies of natural gas has raised new interest in expanding its use in the transportation sector. This report considers issues related to wider use of natural gas as a fuel in passenger cars and commercial vehicles. The attractiveness of natural gas as a vehicle fuel is premised in large part on its low price (on an energy-equivalent basis) compared to gasoline and diesel fuel. When prices for gasoline and diesel are relatively low or natural gas prices are relatively high, natural-gas-based fuels lose much of their price advantage. While natural gas has other benefits-such as producing lower emissions than gasoline and diesel and protecting users of transportation fuels from the volatility of the international oil market-it is largely the cost advantage, if any, that will determine the future attractiveness of natural gas vehicles. There are a number of technology pathways that could lead to greater use of natural gas in transportation. Some require pressurized systems to use natural gas in a gaseous state, and others convert natural gas to a liquid. Two of the most widely discussed options use compressed natural gas (CNG) and liquefied natural gas (LNG). Other technological approaches use liquefied petroleum gas (LPG), propane, and hydrogen. In addition, natural gas can be used to generate electricity to power electric vehicles. Increasing the use of natural gas to fuel vehicles would require creation of an extensive nationwide refueling infrastructure. Although a small number of CNG vehicles have been on U.S. roads for more than 20 years, CNG use has been limited to vehicles that return to a central garage for refueling each day, such as refuse trucks, short-haul trucks, and city buses. LNG, on the other hand, requires large insulated tanks to keep the liquefied gas at a very low temperature and is therefore seen as more suitable for long-haul trucks. In both cases, the limited availability of refueling stations has limited the distances and routes these vehicles may travel. Congress has taken a strong interest in spurring production and use of natural gas vehicles. Legislation has been introduced on a wide range of proposals that would equalize the tax treatment of LNG and diesel fuels, provide tax credits for natural gas vehicles and refueling equipment, require the production of vehicles that could run on several different fuels (such as gasoline and CNG), increase federal research and development on natural gas vehicle tank and fuel line technologies, and revise vehicle emission regulations to encourage manufacturers to produce more CNG passenger cars. Legislation pending in the 113th Congress includes proposals that would extend expired tax credits for refueling property and fuel cell vehicles (S. 2260), authorize the use of energy savings performance contracts to support the use of natural gas and electric vehicles (S. 761), and require the U.S. Postal Service to study the feasibility of using natural gas and propane in long-haul trucks (S. 1486).