To control the quality of indoor air by direct methods requires identification, location, and characterization of suspected pollutants. In this paper, indoor contaminants are characterized as two continua: mass stressors and energy stressors. The rates of generation and transportation of contaminants are described in terms of four sources (outdoor air, building materials, occupants, and indoor processes) in four environmental zones (macro-, meso-, mini-, and micro- environments). In addition to reviewing available information on contaminant generation rates, factors that affect these rates are discussed, including temperature, moisture content, acidity, room air distribution, and occupant proximity. It is concluded in this paper that, even though much is yet to be learned about indoor air quality control, sufficient information is available for new control strategies to be used to provide acceptable indoor air quality in occupied spaces.