Neural tube defects, spina bifida, anencephalus, and encephalocoele, are among the commonest severe congenital abnormalities in most countries. The last decade has seen many advances, including the use of antenatal diagnosis by chemical and ultrasound methods, population screening, and many new findings in epidemiology, culminating in the demonstration of the protective effect of folic acid on recurrences. The progress in both antenatal diagnosis and in aetiological research on neural tube defects has been particularly dramatic, and may well anticipate developments in regard to other congenital abnormalities. Primary prevention now appears as an attainable goal. This book presents a critical review of the extensive findings of epidemiological studies, discusses several methodological issues of wider relevance such as ascertainment, seasonal variation, ethical and legal issues. As well as providing a unique resource in regard to neural tube defects, much of the material is relevant to those with wider interests in the prevention, antenatal diagnosis, and control of major diseases.