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Greenhouse gas and criteria air contaminants associated with motor vehicles are major items in the national inventories of these emissions. While exhaust after-treatment technology has achieved dramatic reductions in the emissions of criteria air contaminants (CO, HC, NOx, PM), alternative technologies for comparable GHG emission reductions are much more challenging. The use of renewable energy forms, or alternative fuels with lower carbon intensity are engineering responses that require careful assessment on a location specific basis before the emission reductions benefits can be accurately assessed. Life-cycle analysis tools such as NRCan's GHGenius model have been developed to serve as analytical frameworks and assist in such assessment. This paper starts by reviewing the issues involved in the life-cycle analysis of alternative transportation technologies, and the available life-cycle tools such as Life Cycle Emissions Model (LEM), GHGenius and Greenhouse Gases, Regulated Emissions and Energy Use in Transportation (GREET) model. The paper then examines the case of compressed natural gas (CNG) vs. diesel from the perspective of a hypothetical Clean Development mechanism (CDM) project for the CNG transit bus fleet in Beijing that was completed as part of the Canada-China Cooperation for Climate Change program.