Prenatal diagnosis provides insight into the in utero evolution of fetal thoracic lesions such as congenital cystic adenomatoid malformation (CCAM), bronchopulmonary sequestration (BPS), congenital lobar emphysema, and mediastinal teratoma. Serial sonographic study of fetuses with thoracic lesions has helped define the natural history of these lesions, determine the pathophysiologic features that affect clinical outcome, and formulate management based on prognosis.