Observational studies and randomized clinical trials demonstrating that antiretroviral prophylaxis of the breastfeeding infant or triple-drug antiretroviral prophylaxis of the lactating mother can significantly affect the risk of postnatal transmission of HIV via breast milk have recently become available. In resource-limited countries, breastfeeding is a cornerstone of infant survival. While shortening the duration of breastfeeding by HIV-infected women reduces postnatal HIV transmission, increasing data suggest this may also decrease overall infant survival. Thus, there is a crucial need for interventions to allow safer and more prolonged breastfeeding. This paper will critically review the results of studies of postnatal antiretroviral prophylaxis to prevent breast milk HIV transmission.