Although research efforts during the past decade have brought about a much better understanding of the relationships between transportation and air quality, much work remains. A better understanding of transportation contributions to key pollutants and of strategies that can be implemented to reduce transportation-related pollution continues to be needed. This paper provides an overview of key air quality and transportation research issues and can serve as a foundation for developing specific national research priorities. Areas discussed include the following: new National Ambient Air Quality Standards for ozone; particulate matter research; global warming and the reduction of greenhouse gases; air toxics; new air quality models; fleet characterization synthesis; project level analysis; new vehicle and fuel standards; alternatively fueled vehicles; institutional relationships between transportation and air quality agencies; strategies to reduce emissions; and sustainable transportation.