Chile has experienced advances in the primary prevention of congenital defects. This article review separately the pre-conception and pre-natal prevention. The first avoids the production of a defective embryo and the latter avoids the effects of external agents, such as environmental teratogens, over a normaly conceived embryo. The preventive measures include education about the use of medications, lifestyles, alcohol and drug use and prenatal control of pregnant women. Special mention deserves nation wide programs held by the ministry of Health such as mandatory rubella vaccination, flour folic acid fortification and metabolic screening of phenylketonuria and congenital hypothyroidism. The role of the main chemical, physical and infectious teratogens is reviewed. A Decalogue of recommendations for primary prevention, elaborated by the Latin American Collaborative Study for Congenital Defects, is included (Rev Méd Chile 2004; 132: 501-8)