Smoking is the principal preventable cause of ill health worldwide.1 It not only affects smokers themselves but is also extremely harmful to non-smokers who inhale environmental tobacco smoke (ETS).2 In non-smoking adults, ETS exposure causes lung and other cancers, ischaemic heart disease, stroke, chronic obstructive pulmonary disease, asthma and other respiratory illnesses.3 Perhaps less well known by the general public is the threat that ETS inhalation poses for infants and children; there are strong associations with children's asthma, lower respiratory tract infection, sudden infant death syndrome, middle ear infection and bacterial meningitis.3 4 Knowledge of the health risks of ETS exposure have led many countries, including the UK, to introduce laws that prohibit smoking in indoor public spaces like bars and pubs (smoke-free legislation).5 Advocates of smoke-free laws successfully argued for these on the basis of protecting non-smokers who might work in or visit smoky environments. Consequently, as tobacco smoke has been completely eliminated from most situations in which adult non-smokers might encounter it, they are well protected from ETS. No similar protection exists for the children of smokers. Most of children's exposure to tobacco smoke occurs domestically3 4 but, internationally, there are no smoke-free laws which forbid smokers from ‘lighting up’ in their homes when children are present. Children's domestic ETS exposure therefore remains an important public health concern which, as Leonardi-Bee and colleagues6 show in this issue of Thorax, is even more harmful than was previously thought. The authors show that children's exposure to ETS from parental smoking has a pervasive inter-generational behaviour-modelling effect such that the children of smokers are much more likely to become smokers themselves. Their systematic review and meta-analysis collated findings from 58 epidemiological studies investigating associations between parental smoking and the subsequent development of …