Mechanisms of teratogenesis are reviewed. The etiology of human malformations occurring during the first year of life is summarized. Genetic and enviromental factors account for 20 to 25 percent and 8 to 10 percent, respectively, of human deformities. Approximately 65 percent of malformations are due to unknown causes. Proposed mechanisms of teratogenesis are summarized. Mutations and chromosome aberrations are discussed. Altered nucleic acid synthesis or function and their role in teratogenesis is considered. Agents causing teratogenic effects in humans are discussed. These include ethyl-alcohol (64175), aminopterin (54626), methotrexate (59052), androgens, coumarin derivatives, diethylstilbestrol (56531), methylmercury (22967926), diphenylhydantoin (57410), oxazolidinediones, polychlorinated biphenyls, progestins, radiation, tetracycline (60548), thalidomide (50351), iodide compounds, radioiodine, and antithyroid drugs. Several infectious agents such as rubella, cytomegalovirus, and syphilis have also been associated with congenital malformations. The authors conclude that it is necessary to continue research on both the etiology and mechanisms of teratogenesis so that avoidable exposures to known and possibly unrecognized teratogens can be prevented in the future.