Because of its abundance and because it offers significant energy and environmental advantages, natural gas has been promoted for use in motor vehicles. A number of transportation fuels are produced from natural gas; each is distinct in terms of upstream production activities and vehicle usage. In this paper, we present greenhouse gas emission impacts of using various natural gas–based transportation fuels. We include eight fuels – compressed natural gas, liquefied natural gas, liquefied petroleum gas, methanol, hydrogen, dimethyl ether, Fischer–Tropsch diesel, and electricity – for use in five types of motor vehicles – spark–ignition vehicles, compression–ignition vehicles, hybrid electric vehicles, battery–powered electric vehicles, and fuel–cell vehicles. In our evaluation, we separate these fuels and vehicle technologies into near– and long–term options to address technology progress over time. Because of great uncertainties associated with advances in both fuel production and vehicle technologies, we establish both an “incremental technology scenario” and a “leap–forward technology scenario” to cover a range of potential technology improvements. Our study reveals that, in general, the use of natural gas–based fuels reduces greenhouse gas emissions relative to use of petroleum–based gasoline and diesel, although different fuels in different vehicle technologies can have significantly different emissions impacts.