Congenital diaphragmatic hernia is a relatively common birth defect. It affects about 1114 babies a year in the United States. Reported survival averages 60% but may be significantly lower. We do not understand the etiology of CDH. Its association with other anomalies and several distinct patterns of presentation suggest that more than one cause may exist. There is a high degree of variability in both treatment and outcomes, but no data exist to allow a rigorous comparison of the efficacy of various treatment strategies. Stratification of patients into more homogeneous groups will be a necessary prerequisite for the design of meaningful comparative trials. The incidence of the lesion prevents any single institution from accruing sufficient patients to conduct such a trial. An ad hoc multicenter study group (the Congenital Diaphragmatic Hernia Study Group) has been formed for this purpose. This organization has begun collecting data with an initial goal of developing a stratification scheme. Prospective data collection should allow verification of several of the estimates made in this article. Current data make it clear that CDH represents a major cause of perinatal morbidity and mortality.