Exposure to environmental tobacco smoke (ETS) is a major source of indoor air pollution and causes adverse effects on the respiratory health of individuals with asthma. At least one third of children and adults with asthma are exposed to ETS on a regular basis. There is convincing evidence for a causal relationship between exposure to ETS and development of asthma in children and in nonsmoking adults. Exposure to ETS also worsens asthma control in children and nonsmoking adults who have established asthma. The mechanism by which ETS causes these harmful effects is not established but may involve genetic predisposition, impairment of lung development, and altered lung inflammatory responses. Workplace smoking restrictions and reduced smoking in the home may lower the prevalence of asthma, improve asthma control, and reduce the use of medical services in both children and adults who are exposed to ETS.