Recent intervention studies have shown that periconceptional supplementation with folic acid-containing multivitamins or pharmacological doses of folic acid alone can reduce the occurrence and recurrence of neural tube defects. This primary preventive method may also reduce the occurrence of other major congenital abnormalities, mainly cardiovascular and urinary tract defects. The underlying biological mechanisms of this protective effect are still not understood, but naturally occurring folates (polyglutamates) or synthetic folic acid (monoglutamate) have a key role.At present, 3 approaches to supplementation with folic acid exist: (i) consumption of a diet that is rich in folate and other vitamins: (ii) periconceptional supplementation: and (iii) fortification of food to ensure appropriate folic acid consumption for all women of childbearing age who are capable of becoming pregnant.The debate over supplementation concerns which vitamins (folic acid-containing multivitamins or folic acid alone?) and what dosages (0.4, 0.8 or 4 to 5 mg/day of folic acid?) are to be given to whom (is it worthwhile differentiating between women at high and low risk?).