• The development of new knowledge and new diagnostic techniques and technology as well as the sophistication of epidemiology studies and maturation of the fields of clinical genetics and clinical teratology have revolutionized the field of reproductive and developmental biology.• Advances have enabled physicians and scientists to determine the causes of developmental abnormalities and, therefore, discover methods of prevention. The process of evaluation is based on the knowledge base developed over the past 50 years.• Although genetic abnormalities are responsible for a significant proportion of reproductive and developmental deleterious effects, a larger proportion of these effects are due to unknown causes.• Environmental causes are less frequent, although many of the environmental effects as well as many of the genetic effects can be prevented through genetic counseling and preconceptual planning. Effective treatment and amelioration of developmental effects also have improved.• More than 50 environmental drugs, chemicals, maternal diseases, infections, nutritional abnormalities, and physical agents can affect reproduction deleteriously and result in CMs.Theoretically, all these causes are preventable.• Throughout the developing world, the addition of folic acid and iodine could prevent tens of thousands of birth defects and developmental abnormalities.• In the United States, the opportunity for prevention can be introduced at the population level and by addressing individual patients’ clinical problems.• If a mother of a malformed infant had some type of exposure during pregnancy, such as a diagnostic radiologic examination or medication, the consulting physician should not support or suggest the possibility of a causal relationship before performing a complete evaluation. If a pregnant woman who has not yet delivered had some type of exposure during pregnancy, the consulting physician should not support or suggest the possibility that the fetus is at increased risk before performing a complete evaluation. • Every patient deserves a complete, scholarly evaluation that uses the basic principles of teratology and risk analysis.