Modern neonatal practice has improved the outcome of extremely preterm infants. However, why some infants require prolonged periods of respiratory support while others improve after a short period of mechanical ventilation, remains largely speculative. Many risk factors, including barotrauma or volutrauma due to mechanical ventilation, oxygen toxicity, and infection, have been identified for the development of chronic lung disease of prematurity (CLD). Attempts to minimise these with modern neonatal practice, including newer ventilatory techniques, have had minimal impact on its incidence. Factors other than barotrauma and oxygen toxicity are likely to be important in the development of CLD.

Although our understanding of normal fetal lung development has increased substantially over the past few years, it nevertheless remains rudimentary, especially in infants who have survived neonatal intensive care. Animal models have provided many clues to the effects of interventions in the neonatal unit on the lung growth of preterm infants. Normal lung growth and some of the abnormalities that may result from disordered growth or from medical interventions are reviewed in this article. There are a vast number of other factors which influence lung growth—some, such as fetal breathing and lung fluid dynamics, deserve reviews of their own.