The transportation sector accounts for a large and growing share of global greenhouse gas (GHG) emissions. Worldwide, motor vehicles emit well over 900 million metric tons of carbon dioxide (CO2) each year, accounting for more than 15 percent of global fossil fuel-derived CO2 emissions. In the industrialized world alone, 20-25 percent of GHG emissions come from the transportation sector. The share of transport-related emissions is growing rapidly due to the continued increase in transportation activity. In 1950, there were only 70 million cars, trucks, and buses on the world's roads. By 1994, there were about nine times that number, or 630 million vehicles. Since the early 1970s, the global fleet has been growing at a rate of 16 million vehicles per year. This expansion has been accompanied by a similar growth in fuel consumption. If this kind of linear growth continues, by the year 2025 there will be well over one billion vehicles on the world's roads. In a response to the significant growth in transportation-related GHG emissions, governments and policy makers worldwide are considering methods of addressing this trend. However, due to the particular make-up of the transportation sector, regulating and reducing emissions from this sector poses a significant challenge. Unlike stationary fuel combustion, transportation-related emissions come from dispersed sources. Only a few point-source emitters, such as oil/natural gas wells, refineries, or compressor stations, contribute to emissions related to the transportation sector. The majority of transport-related emissions come from the millions of vehicles traveling the world's roads. As a result, successful GHG mitigation policies must find ways to target all of these small, non-point source emitters, either through regulatory means or through various incentive programs. To increase their effectiveness, policies to control emissions from the transportation sector often utilize indirect means to reduce emissions, such as requiring specific technology improvements or an increase in fuel efficiency. Site-specific project activities can also be undertaken to help decrease GHG emissions, although the use of such measures is less common. These activities include switching to less GHG-intensive vehicle options, such as natural gas vehicles (NGVs). As emissions from transportation activities continue to rise, it will be necessary to promote both types of abatement activities in order to reverse the current emissions path. This Resource Guide focuses on site- and project-specific transportation activities. To date, only a few projects deploying NGV technologies have been developed and implemented with the explicit intent of reducing GHG emissions and participating in international GHG reduction initiatives. Therefore, experience with quantifying, evaluating, and verifying GHG emission reductions from natural gas vehicle projects is almost non-existent. This is a problem as there are many issues unique to the transportation sector, which should be resolved before adequate guidelines can be developed for evaluating transportation-related projects. Issues that will require further analysis and guidance include: 1. Methods for accurately estimating emission reductions for a dispersed number of sources; 2. Procedures for determining project boundaries and relevant GHG emission sources; 3. Options for minimizing transaction costs of validating, monitoring, verifying, and certifying potential emission reductions; and 4. Guidance on using a full fuel-cycle or tailpipe emission analysis to estimate project emissions. The main purpose of this manual is to provide information on quantifying and documenting GHG emission reductions from NGV projects. Moreover, to provide potential project developers with an overview of project opportunities, the manual also includes information on NGV technology cost and availability and discusses the future of the alternative fuel vehicle (AFV) industry as a whole.