While extensive measurements have been and are being made of outdoor pollution, very few data have been gathered on indoor pollution. The data that are available are compiled and analyzed in the report. Based on a review of the literature, it was possible to infer relationships between indoor and outdoor pollution and to identify factors that affect these relationships. The relationships identified must be considered tentative. Except for bacteria and perhaps, for fungus spores, indoor pollution levels appear to be controlled primarily by outdoor concentrations. Other factors that influence indoor pollution levels include internal activities and pollutant generation, atmospheric conditions and natural ventilation, time, location, type of building, and air conditioning and filtration systems. Indoor concentrations of pollen and reactive gases, expressed as a percentage of outdoor concentrations, decrease with increasing outdoor concentrations. Bacterial concentrations indoors appear to be more closely related to the presence and activities of people inside than to outdoor concentrations.