The transportation sector accounted for almost 28% of total U.S. greenhouse gas (GHG) emissions in 2004, according to Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004, published in 2006 by the U.S. Environmental Protection Agency (EPA). Over the period 1990 to 2004, GHG emissions from the transportation sector increased at the fastest rate of any end-use economic sector in the United States and accounted for the largest absolute increase of any of these sectors. The largest sources of U.S. transportation GHG emissions continued to be lightduty vehicles, heavy-duty trucks, and aircraft. While these three modes demonstrated increases in travel activity from 1990 to 2004, their GHG growth rates differed considerably: GHGs from heavy-duty trucks increased by 62%, light-duty vehicles by 23%, and commercial aircraft by only about 10%. This paper (a) analyzes factors affecting transportation GHG emissions in the United States, including the output from specific modes and travel purposes; (b) examines sources of uncertainty in these estimates; and (c) describes methodological improvements used in EPA's 1990-2004 inventory and other planned improvements.