Most causes of hearing loss in newborn infants are hereditary and cannot be prevented. However, about 30% of cases of hearing loss in newborns have been linked to teratogenic factors, and many of these cases are preventable. Teratogens are factors capable of causing physical defects in the developing fetus or embryo and are typically grouped into four categories: infectious, chemical, physical, and maternal agents. During intrauterine life, the fetus is protected from many teratogens by the placenta, which serves as a filter to prevent the toxic substances from entering the fetus' system. The placenta, however, is not a perfect filter and cannot prevent entry of all teratogens. Prenatal susceptibility to teratogens and the severity of the insult are quite variable. Four factors believed to contribute to this variability are dosage of the agent, the timing of the exposure, the susceptibility of the host, and interactions with exposure to other agents. This entry discusses teratogenic chemicals that contribute to hearing loss in the newborn. It should be kept in mind that much is still unknown about chemical teratogens and their ultimate impact on the developing auditory system.