Since the inception of the Clean Air Act (CAA) in 1969, atmospheric models have been used to assess source-receptor relationships for sulfur dioxide (SO2), CO, and total suspended particulate matter (TSP) in the urban areas. The focus through the 1970's has been on the Gaussian dispersion models for non-reactive pollutants. The 1977 Amendments to the CAA mandated the use of dispersion models for assessing compliance with the relevant National Ambient Air Quality Standards (NAAQS) when new sources of pollution are permitted and for prevention of significant deterioration. In the 1980's, the focus has been on the secondary pollutants (e.g. ozone, acid rain), which led to the development of grid-based photochemical models to better understand the urban and regional scale pollution. In the 1990's, attention was paid to the development of one-atmosphere models to deal with multiple pollutants. The new NAAQS for ozone and fine particulate matter PM2.5 that were promulgated in 1997 call for the use of one-atmosphere models in designing multi-pollutant emission control strategies. In the 2000's, there is considerable interest in the development of integrated air shed-watershed models to properly assess the effects of atmospheric pollution on sensitive ecosystems.