Gross anomalies of structure and/or function affect 2 to 4 per cent of all human newborns and are the leading cause of perinatal mortality in this country. Despite their significance, the etiology of most such defects remains unclear. A majority are unassociated with any identifiable cause; a small but significant percentage are atrributed to heritable disorders of either a Mendelian (single gene) or chromosomal nature. Drugs and environmental exposures are currently implicated in only a small percentage of affected pregnancies. Nevertheless, an awareness of the principles of teratogenesis, an appreciation for the role - though imprecise - of placental transfer and fetal drug disposition, and avoidance of known teratogens currently offers our best hope for the study and prevention of birth defects.