This proceedings, ""Transportation Planning and Air Quality III"", consists of papers presented at the conference held in Lake Tahoe, California, August 17-20, 1997. It represents a significant step toward the dissemination of state-of-the-art practices and understanding of transportation and air quality issues. These papers present findings of a host of scientific investigations as well as air-quality case studies undertaken by various stakeholder groups. In addition, they discuss various analytical tools and methods for structuring air quality analyses to better capture real world emissions. This proceedings represents a major milestone on the path toward understanding the interaction between transportation and air quality.