In low and middle-income countries (LMIC), transmission of HIV during breastfeeding represents a major public health challenge. Several viral, maternal clinical, immunological and genetic factors, as well as maternal-infant host factors and type of infant feeding may influence the risk of breastfeeding transmission of HIV. The mechanisms of breast milk HIV transmission are poorly understood. For mothers who are healthy and do not need combination antiretroviral therapy for their own health, randomized controlled trials have proven that administration of extended maternal triple-drug antiretroviral (ARV) prophylaxis or extended infant ARV prophylaxis can significantly reduce the risk of HIV transmission during breastfeeding. Based on this evidence, the World Health Organization (WHO) published new guidance in 2010 on the use of ARVs for treating pregnant women, and preventing mother-to-child HIV transmission (PMTCT). Although, remarkable advances have occurred in prevention of postnatal transmission during breastfeeding using antiretroviral strategies, a number of challenges remain. Future research must focus on field studies to evaluate programmatic implementation of new WHO PMTCT regimens, monitor long-term safety of ART exposure during pregnancy and lactation, and study emergence of ARV resistance (in mothers and infected infants despite prophylaxis).