A teratogen is any environmental agent that permanently harms the developing fetus. Health-care providers who care for pregnant women are consulted regarding a variety of agents, including prescribed or over-the-counter medications, vaccinations, diseases or infections, chemical agents in the work place or at home, radiation, and alcohol, tobacco, and "street drugs." Patients also may have questions about the impact of paternal exposures on subsequent offspring and the safety of maternal use of medications while breastfeeding. Thus, it is important for the caregivers to be knowledgeable concerning basic principles of teratogenesis, such as the effects of timing, dosage, and fetal susceptibility, and the impact of specific agents on the fetus or pregnancy. Clinicians should be cognizant of the possibilities for prenatal diagnosis of fetal damage, strategies to decrease risk, and available resources.