Asthma is a frequently occurring disease with an increasing prevalence worldwide, that is the highest in childhood. Smoking, both active and passive, is also a widespread problem. Epidemiological and experimental studies show that active and passive smoking have an important influence on the severity and the development of asthma. There is sufficient evidence to conclude that there is a relationship between passive smoking and both the exacerbation and the development of asthma in children. In adults, there is sufficient evidence for a relationship between passive smoking and the exacerbation of asthma, but there is only suggestive evidence for the relationship between passive smoking and the development of asthma. There is sufficient evidence for a relationship between active smoking and the exacerbation of asthma in anybody who smokes. Besides, active smoking impairs the efficacy of inhaled corticosteroid treatment in asthma. There is no evidence for a causal relationship between active smoking and the development of asthma. Children of mothers who smoked during pregnancy or young children who are exposed to environmental tobacco smoke, are at the highest risk to develop asthma during their life. Consequently, every physician plays an important role in helping pregnant women, parents of young children and asthmatic patients (and as a matter of fact every smoker) to stop smoking.