In recent years, environmental tobacco smoke (ETS) has become an important worldwide public health issue. Children are particularly vulnerable to ETS because they are still developing. ETS exposure causes a wide range of adverse health effects on childhood asthma. There is convincing evidence that ETS exposure is causally associated with an increased prevalence of asthma, increased severity of asthma and worsening asthma control in children who already have the disease, even though a causal relationship with asthma onset is not yet established for asthma incidence. Mechanisms underlying these adverse effects of ETS are not clearly elucidated but e studies on this issue suggest that genetic susceptibility, impaired lung function, and augmented airway inflammation and remodeling may be involved. Children with asthma are just as likely to be exposed to ETS as children in general and there is no risk-free level of exposure. Therefore, providing a smoke-free environment may be of particular importance to the asthmatic children exposed to ETS who have adverse asthma outcomes, as well as to children with genetic susceptibility who are at increased risk of developing asthma upon exposure to ETS in early childhood.