This work discusses the theory of the HIWAY and California Line Source highway air pollution dispersion models and describes the EPA emissions model which is used to provide emission factors for these models. A parametric study of the dispersion models is performed to show the effect of the various inputs to these models on predicted pollutant concentrations. These results indicate certain cases in which one model should be used instead of the other. These models are used to perform air quality, environmental impact assessments of two highway projects in Massachusetts. Mesoscale Analyses are also performed for these highways. Advances in modeling the dispersion of pollutants from highways and possible rnodifications to the California Line Source and HIWAY models are also discussed.