Given the preponderance of evidence showing that diesel fumes are more toxic than previously known, there has been greater interest in substituting diesel with natural gas for vehicles operating in cities where there is greater air pollution. This report examines whether natural gas vehicles are an important part of solving this problem. Transit buses, traditionally fueled by diesel, are one of the cheapest forms of mass transit. They are also significant polluters and typically operate in heavily congested urban areas, where significant air pollution problems exist. The report provides an overview of the issues that must be considered when evaluating natural gas an alternative to diesel for use in transit buses.