By far the largest category of malformations, 65% falls into the group of those with an unknown cause(s). Purely genetic causes of malformations (autosomal and cytogenetic), estimated to produce 20 to 25% of all human malformations, comprise the largest group of congenital malformations with known etiology. Although environmental causes of human malformations account for 10% or fewer of malformations, most of these environmentally induced malformations are related to maternal disease states. Fewer than 1% of all human malformations are related to drug exposure, chemicals, or radiation, but studies of environmentally induced malformations are important because they may teach us how to predict and test for teratogenicity, understand the mechanisms of teratogenesis from all etiologies, and provide a means by which human malformations can be prevented.