We highlight evidence demonstrating antenatal smoking exposure is an important risk factor for increased respiratory symptoms and lung function abnormalities in infants and children. Epidemiological studies have demonstrated an excess both of wheezing in the first two years after birth and asthma and persistent wheezing in older children. Lung function testing in children exposed to antenatal smoking has demonstrated a reduction in airway function. Antenatal exposure of nicotine to animal models results in pulmonary hypoplasia, fewer but larger alveoli and altered airway morphology. Pulmonary function testing, however, has not demonstrated that infant lung volume is affected by antenatal smoking exposure, other than due to the expected effect of smoking on somatic growth, but there is an adverse effect on airway development. There is no evidence that antenatal smoking exposure increases bronchial hyperreactivity, rather it may be associated with a diminished response to both bronchoconstrictors and bronchodilators in infants.