Mother-to-child transmission of HIV through breast-feeding is the remaining challenge facing mothers in resource-poor settings with a high HIV prevalence. Nearly all infants in developing countries are initially breast-fed, and most children continue to receive some breast-feeding until at least 6 months of age but frequently into the 2nd year of life, especially in sub-Saharan Africa and Asia. In December 2002, international researchers convened in Ghent, Belgium, to discuss mechanisms for, rates and risk factors of, and approaches to prevention of HIV transmission through breast-feeding. Four papers were compiled bringing together the presentation and discussions during this workshop, while the fourth paper also benefits from presentation made during an earlier workshop on vaccines in the prevention of mother-to-child transmission. These papers summarize the current state of knowledge and highlight the outstanding issues that will need to be addressed in the very near future before research advances can be translated into public health practice.