Over the last half of the last century, classic studies established many features of the development of the lung in humans. Early studies relied on meticulous measurement and evaluation of lung morphology and physiology. In recent years, such studies have been supplemented by carefully conducted, longitudinal studies and by the use of molecular biological methodologies to determine more detailed aspects of lung development. In addition, factors affecting the growth of the lung have been evaluated including preterm delivery, antenatal steroids, environmental factors, asthma and asthma medications. In the future, further advances are likely to come from the molecular biological approach.