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Knowledge of the development of the lungs helps us to understand congenital pulmonary anomalies. The lungs develop after 22 to 24 weeks with the ultimate purpose of sustaining life after delivery of the newborn. This occurs by oxygen from the atmosphere being inhaled into the lungs and carbon dioxide being exhaled. Oxygen provides the fuel to allow the human body to function. In this chapter, the gross anatomy of the lung, rather than microscopic anatomy, is described. The 3 laws of development of the lungs according to Hislop and Reid1 are as follows: 1. The bronchial tree is developed by the 16th week of intrauterine life. 2. Alveoli develop at 28 weeks, increasing in number until 5 to 8 years of age, and increasing in size until growth of the chest wall finishes with adulthood. 3. The pre-acinar vessels (arteries and veins) follow the development of the airways; development of the intra-acinar vessels follows that of the alveoli. Muscularization of the intra-acinar arteries does not keep pace with the appearance of new arteries. There are 5 phases or periods of lung development. Before 3 weeks of gestation, a pouch arises from the primitive foregut, at which time the embryo is 3 mm long. This period constitutes the embryonic phase. The lung bud divides into right and left, which will become the right and left lungs. By the end of the embryonic period, there will be 5 additional branches, which are the major bronchi, and the lung buds will have elongated into primary lung sacs. Developmental anomalies at this stage will result in anomalies of the major airways, including atresia (closed or undeveloped) and tracheoesophageal fistula.