Teratogenic exposures are those that can cause an embryo or fetus to develop abnormally. Several factors determine whether an agent is teratogenic, including the gestational timing of the exposure, as well as the dose, route and nature of the agent itself. We review the general concepts of teratogenesis, as well as known genetic susceptibilities to teratogenic effects, with a special focus on antiepileptic drugs. We discuss general principles of risk counselling and risk reduction, and we describe several long-known teratogens, as well as several exposures recognized only recently to have teratogenic potential.