Strong observational and experimental evidence indicates that lung growth during fetal and early postnatal life is one of the strongest determinants of adult lung function. Genetic variation plays a critical role in determining maximal lung function reached in adult life. Factors that affect lung growth such as extreme prematurity with bronchopulmonary dysplasia, intrauterine growth retardation, exposure to tobacco smoke in utero and postnatally, and vitamin A and D deficiencies also play varying roles in determining lung function. However, catch-up growth seems to be able to reverse at least in part the negative effects of some of these conditions. Individuals who reach early adult life with lower levels of lung function are at increased risk of developing chronic obstructive pulmonary disease during the decline phase of lung function, after the third decade of life, and may also be more susceptible to the deleterious effects of active cigarette smoking.