Epidemiologic studies have established clear variations in the occurrence of neural tube defects (NTDs) associated with demographic and other factors. Methods of counting NTD-affected pregnancies and other birth defects rely on multiple sources of information such as hospital records and birth certificates. Widespread prenatal diagnosis of NTDs presents special challenges for birth defects monitoring programs. Even before prenatal diagnosis was possible, epidemiologists noted geographic variation in NTD rates; distinctive recurrence patterns within families; variations in NTD rates by race/ethnicity, socioeconomic strata, and sex of the infant or fetus; and changing rates over time. We review highlights of studies of these factors and some conclusions that can be drawn from NTD epidemiologic data. These conclusions include the importance of maternal diet, a finding that has led to preventive interventions, and the suggestion of multiple causes of NTDs, indicating that no single prevention program may prevent the occurrence of all NTDs. We also discuss the relevance of epidemiologic data to NTD screening and counseling policies and the importance of prenatal diagnosis data. Future epidemiologic studies will need to incorporate prenatal records, particularly to study trends. Other emerging themes in epidemiologic research include collaboration between different birth defects monitoring programs, new information from unique geographic areas, new techniques to study gene-environment interactions, and a multidisciplinary emphasis on measuring the effectiveness of prevention programs.