Teratology has grown from a little known discipline to a discipline embracing a vast accumulation of literature on experimental studies in many animal forms. Emphasis has shifted from preoccupation with descriptions of anatomical defects to concern about subtle and interacting causative factors. The aim of this work is to assemble a source of facts, concepts, methods and references within the broad scope of teratology. Our review summarizes reports of teratogenic effects in mammals. The major intent of this work, however, has been to introduce into a field of teratology. Special attention has been paid to the relation of animal studies to the problem in man and to the interactions between teratogens and environmental variables. We present a number of illustrative examples of various kinds of interactions that may occur and their possible significance. Striking degrees of potentiative interactions have been demonstrated in laboratory animals when two or more compounds were used simultaneously at doses at or below the threshold level for teratogenicity of the same compound when applied singly. We propose the name coteratogenesis for the agents which are not teratogenic but which aggravate teratogenicity of the other factors known as teratogens.