A number of urban factors are relevant to the problem of defining air quality control region boundaries. First, the location of population is an important consideration, since human activity is the primary cause of air pollution, and humans are the ultimate victims. The projected population growth pattern is another important consideration, since an air quality control region should be designed not only for the present but also for the future. The location of industrial activity and the political and jurisdictional considerations are important. The reports discussion of urban factors will present these considerations as they apply to the Puget Sound area.