Among asymptomatic healthy infants decreased airway function is found in infants exposed to environmental tobacco smoke (ETS) or with history of family asthma (Fm Asthma), or male gender. These infants are at significantly greater risk of wheezing during infancy. These findings indicate that genetic and environmental factors affect the mechanical determinants of airway function and that among healthy infants variations of these mechanical determinants contribute to the increased risk of developing airway obstruction severe enough to result in clinical symptoms of wheezing. The mechanisms of reduced airway function in these at risk infants are not known. In adults reduced airway function is a result of airway narrowing, increased airway compliance, or decreased pulmonary elastic recoil. The same is thought to be true in infants. The cause of airway narrowing may be functional (reversible by bronchodilators) or structural (fixed, not reversible by bronchodilators) or have components of both. Fixed determinants of airway narrowing include small absolute airway size, thickened airway wall, decreased elastic recoil, and small airway size relative to lung volume. Reversible determinants include airway hyper-responsiveness and inflammation. Understanding the alterations of airway function that increase the risk of airway disease will be decisive in the development of strategies for the prevention and treatment of wheezing in infants and children, and hence, a reduction of morbidity and mortality.