Genetic causes account for 20% to 25% of human birth defects, but the largest proportion of birth defects have no definitive etiology and some of these malformations may be due to intrinsic, “nonpreventable” spontaneous errors of development. Environmental causes, which include maternal disease states, maternal infection, mechanical factors, problems of constraint, chemicals, drugs, and physical agents, are responsible for only about 10% of human birth defects. The scientific basis for understanding the risk of congenital malformations from exposure to environmental agents is based on several tenets of toxicology and embryology dogma. The first tenet is that essentially all teratogens that have been studied have a typical toxicologic dose response relationship and a no-effect dose. Secondly, the stage of gestation is critical to the effects that are expected, and all stages of embryogenesis and fetogenesis can have vulnerability to environmental toxicants. Thirdly, the response of the embryo and fetus is characteristic for each teratogenic agent, although there is some similarity in the effect of certain teratogens. Appropriately designed developmental toxicology studies and basic embryologic and biologic concepts are all used to estimate the potential reproductive hazard for embryonic death, growth retardation, congenital malformation, and functional deficit.