Background: Estimates of Paediatric HIV are essential for planning national HIV programs. Although there is a large amount of empirical data on the prevalence of adult HIV from antenatal clinics and national surveys there is very little HIV data for children, necessitating estimates based on knowledge of: HIV infection in pregnant women; transmission rates among pregnant and breastfeeding women according to their treatment status; and survival of infants infected in different ways. It is essential that these inputs into estimating paediatric HIV are as accurate as possible as there is little empirical data to calibrate the final estimates of prevalence of paediatric HIV. Currently there are gaps in the understanding of some of the inputs needed to estimate paediatric HIV and a potential to improve estimates as new data become available, particularly as more widespread availability of antiretroviral treatment changes the circumstances in which children become infected. Aims and Objectives: The aim of the research is to improve and fill gaps in knowledge about the HIV epidemic and thereby improve estimates of paediatric HIV. Objectives include improving estimates of survival of infected children, exploring the acquisition of HIV by women in relation to incidence during pregnancy, furthering understanding of the impact of HIV on fertility and understanding the biases inherent in different data sources. Implications: The new empirical evidence and rigorous methods developed to evaluate the inputs needed to estimate the number of children born to HIV positive women and the prevalence of paediatric HIV will produce more reliable HIV epidemic projections, and will improve information available to policy makers and programme planners.