The promotion of nearly universal breastfeeding has played an important role in improving child health by providing optimum nutrition and protection against common childhood infections, and by promoting child spacing. Unfortunately, it has become clear that breastfeeding is responsible also for much of the increasing burden of worldwide pediatric human immunodeficiency virus (HIV) infection, especially in the developing nations (12–14% additional risk of HIV infection transmitted by breastfeeding; 35% total proportion of all HIV-infected children in an area infected through breastfeeding). Several factors influence the transmission of HIV by breastfeeding, including whether a woman acquires her infection during breastfeeding (29% risk of transmission) or before pregnancy (7–10% risk of breastfeeding transmission),the degree of maternal plasma and breastmilk viral load, and the presence of mastitis. In areas of the world where adequate sanitary replacement feeding is not available, the decision to withhold breastfeeding so as to decrease HIV transmission may lead to increased rates of child morbidity and mortality from diarrheal and respiratory diseases, and malnutrition. This review summarizes current data on the pathophysiology of breastfeeding transmission of HIV infection, the risk factors for and incidence rates of transmission, and the feasibility of possible alternatives to exclusive breastfeeding in the setting of maternal HIV infection. Clearly, women must be fully informed about the risks of breastfeeding transmission of HIV, the risks of morbidity and mortality among nonbreastfed infants, and the expense and availability of procuring adequate replacement formula. If an uninterrupted access to a nutritionally adequate breastmilk substitute that can be safely prepared is ensured (as is possible in industrialized countries), HIV-infected women should be counseled not to breastfeed their infants.