The purpose of this chapter is to provide an understanding of the differences between and similarities of indoor air quality given that most people spend more time in the indoor environment that the outside in the ambient air. This chapter provides an understanding of how pollutant concentrations differ in indoor environments as compared to the ambient environment, and understanding of how personal pollutant exposures differ from ambient exposures. It also provides information on specific pollutants including the past use and nature of asbestos use in buildings, asbestos-related disease, and potential risks of indoor exposures; awareness of how combustion devices and activities contaminate indoor air and understanding of potential health risks associated with such exposures; understanding of the nature of aldehyde (particularly formaldehyde) emissions and exposures in buildings and potential health effects; recognizing the nature of indoor contamination of buildings by volatile and semivolatile organic compounds (including pesticides), exposures, and potential health effects; becoming familiar with the significant role that a variety of biological contaminants play in causing illness/disease in building occupants on exposure; becoming familiar with sick/problem building phenomena, building-related illness, and sick building syndrome; understanding of approaches used to control indoor pollutants such as source management, ventilation, and air cleaning; and understanding of regulatory and nonregulatory initiatives and public policy concerns related to indoor air quality.