Breastfeeding accounts for up to half of all infant human immunodeficiency virus (HIV) infections worldwide and carries an estimated transmission risk of about 15% when continued into the second year of life. Because replacement feeding is not safely available, culturally acceptable, or affordable in many parts of the world and because breastfeeding provides protection against other causes of infant mortality, approaches that reduce breastfeeding mother-to child transmission of HIV are being explored. These include exclusive breastfeeding for the infant's first few months of life followed by rapid weaning, treatments of expressed milk to inactivate the virus, and antiretroviral prophylaxis taken by the infant or mother during breastfeeding, which are strategies currently being tested in clinical trials. Passive (antibodies) and active (vaccine) immunoprophylaxis will also soon begin to be tested. This paper focuses on current and planned research on strategies to prevent breastfeeding transmission of HIV.