There is still no clear understanding of what causes the great majority of human congenital malformations. And since in most sorts of human disease and pathology that yet prevail prevention usually awaits understanding of cause, it is generally thought that the same is true of developmental aberrations. But is this true? For the relatively few congenital malformations whose causes are primarily environmental, it is plain that their discovery has enabled prevention, but not nec­ essarily immediately. It took a generation from the time of the discovery that maternal rubella was teratogenic to learn how to immunize against it. Much debate occurred before it was appreciated that thalidomide was a teratogen, and only its removal from the pharmacist's shelf and the end of the epidemic of limb defects attributed to the drug overcame the last doubts. For other proven environmental teratogens doubts and difficulties still con­ tinue. The claimed prevalence of fetal genital distortions due to female sex hor­ mones may have been exaggerated. Some potentially teratogenic therapeutic drugs, like anticoagulants, anticonvulsants, and anticancer chemicals, are still pre­ scribed despite this danger because of their benefits to pregnant women. For those congenital malformations whose basis is predominantly genetic or chromosomal it is different, however. Prevention has not been achieved by the discovery of such causes, as dramatic and revolutionary as some of them have been, except in the questionable sense of interference with reproduction by genetic coun­ seling or prenatal elimination. But this has not inhibited the romanticists.