This article reviews the pathophysiology of congenital defects and tumors that are of interest to the pediatric surgeon. Reliable information has been obtained through prenatal diagnosis of fetal anomalies and careful studies of fetal animal models. The topics covered include fetal diaphragmatic hernia, oligohydramnios-induced pulmonary hypoplasia, renal dysplasia, prune belly, gastrointestinal obstruction, biliary atresia, Wilms' tumor, and neuroblastoma. In addition, some recent experimental studies delineating the ability of the fetus to heal without scarring may have implications for all surgeons.