Mother-to-child transmission of human immunodeficiency virus type 1 (HIV) occurs during gestation, during delivery, and during breast feeding. In an unprotected mother-child pair, transmission over-all occurs in 30-40%, with about one quarter of these transmissions in utero, one half during delivery, and one quarter during breast feeding. Most in utero transmission occurs in the third trimester. There are many risk factors for transmission, but the most important are the maternal viral load and the maternal CD4 concentration. Antiretroviral treatment of the mother has a potent preventive effect but must be administered throughout the risk period (that is, from early second trimester through the end of breast feeding). For in utero and intrapartum transmission, treatment probably acts through two mechanisms, namely pre-exposure prophylaxis in the fetus or newborn, and reduction in maternal viral load. Adequate voluntary counseling and testing for HIV and access to antiretroviral drugs are now critical preventive issues in this important mode of transmission.