Energy supply and environmental concerns have led to interest in alternative transportation fuels and power-trains. Already, there are significant changes in mainstream gasoline and Diesel formulation to accommodate tighter emissions standards. Some alternative fuels are being promoted as “cleaner” replacements for gasoline and Diesel fuel. There are many research papers which present data on these different alternative fuels, yet it is difficult to compare the fuels with any confidence. The majority of published studies do not use consistent methodology and make many assumptions (which may or may not be reported). Based on an extensive literature review, this study presents emissions results drawn from a smaller number of papers which provide alternative fuel and conventional emissions data in a comparable manner. Both light-duty and heavy-duty vehicles are considered. Reformulated gasoline, compressed natural gas, liquified petroleum gas, methanol-85 and methanol-100 are compared to conventional gasoline and Diesel fuels. The scope of the study includes emissions comparisons on the basis of standard emissions test cycles, low ambient temperature effects, mileage degradation as well as vehicle technology. Additionally, some of the parameters producing variances in emissions values from paper to paper are discussed.