Passive smoking is the inhalation of smoke, called secondhand smoke (SHS) or environmental tobacco smoke (ETS), from tobacco products used by others. It occurs when tobacco smoke permeates any environment, causing its inhalation by people within that environment. Scientific evidence shows that exposure to secondhand tobacco smoke causes disease, disability, and death.

Passive smoking has played a central role in the debate over the harms and regulation of tobacco products. Since the early 1970s, the tobacco industry has been concerned about passive smoking as a serious threat to its business interests; harm to "innocent bystanders" was perceived as a motivator for stricter regulation of tobacco products. Despite an early awareness of the likely harms of secondhand smoke, the tobacco industry coordinated to engineer a scientific controversy with the aim of forestalling regulation of their products. Currently, the health risks of secondhand smoke are a matter of scientific consensus, and these risks have been one of the major motivations for smoking bans in workplaces and indoor public places, including restaurants, bars and night clubs.