Environmental tobacco smoke (ETS) contaminates indoor air in homes and workplaces. Although the adverse effects of active cigarette smoking on the respiratory tract have been extensively characterized, the effects of ETS exposure on adult asthma have not yet been investigated extensively and the available data are limited. This article examines the evidence for ETS exposure as a cause of asthma and asthma exacerbation in adults, and for ETS exposure in the workplace specifically as contributing to these health effects. It addresses methodological barriers that limit the available data and evaluates the adequacy of the data for risk assessment.