This report provides framework and guidance for estimating the life cycle greenhouse gas emissions for transportation fuels, specifically aviation fuels. The focus on aviation fuels was driven by the patterns of fuel use by the federal government. Policies such as those outlined in Section 526 of EISA 2007 cause federal agencies to institute enforceable guidelines for procuring low carbon alternative fuels. Federal consumption of fuels is dominated by the Department of Defense and the Air Force consumes more fuel than any of the other military services or federal agencies (Defense Science Board 2008). Thus, aviation applications may become early adopters of low carbon transportation fuels. The U.S. Air Force convened a working group of individuals from government agencies, universities and companies actively engaged in assessing greenhouse gas emissions from transportation fuels, and requested that this group develop guidance on procedures for estimating greenhouse gas emissions in aviation applications, using currently available data and tools.