Since the early 1970s, the health effects of indoor air pollution have been investigated with increasing intensity. A large body of literature is now available on diverse aspects of indoor air pollution: sources, concentrations, health effects, engineering, and policy. This article provides a selective summary of this new information with an emphasis on health effects relevant to health care practitioners concerned primarily with immunologically mediated respiratory diseases. We address exposures associated with acute and chronic respiratory effects: tobacco smoke, nitrogen dioxide, wood smoke, and formaldehyde. The article also describes the diverse health problems experienced by workers in newer sealed office buildings. The importance of indoor concentrations in determining personal exposures to pollutants is emphasized.