One of the most remarkable successes of epidemiology was the demonstration in the late twentieth century that spina bifida and anencephaly-two of the most common and severe birth defects-are caused primarily by folate deficiency. This article reviews the descriptive epidemiological studies that began when we did not have a clue about etiology. The paper tells the success story of the trials that proved that folic acid would prevent folic-acid-preventable spina bifida. Finally, it will tell how difficult it is to get prevention policy implemented, even when the scientific evidence is compelling. It concludes by noting that the inaction or inappropriate actions of food regulatory bodies in so many countries means that only 10% of folic-acid-preventable spina bifida is actually being prevented--a serious failure of public health policy.