Summary The impact of the HIV/AIDS pandemic on societies is without precedent in recorded human history. Globally, 40 million people are living with HIV/AIDS, the vast majority in sub-Saharan Africa. A prerequisite for the development and implementation of prevention interventions is an understanding of who is at risk and why. Many characteristics of the individual, the community they are from and the infection itself contribute to determining the risk of exposure and acquisition and multiple causal pathways exist linking social, demographic, economic, cultural and behavioural variables. Whilst many advances have been made in understanding the spread of HIV from studies of individual risk factors in a diverse range of populations, much is still unknown about why populations have experienced different epidemics, and what the drivers of high risk are compared with variables that simply correlate with risk. The project will use a multi-level framework to better understand the causal pathways of HIV infection (both in terms of acquisition and transmission of infection) in a rural South African setting where population-based surveys have measured HIV prevalence at >50% in some age-groups. The research aims to identify and quantify important environmental, community, household and individual-level determinants of HIV incidence and prevalence to inform intervention strategies. Particular emphasis is placed at the level of the local community which is seen as being vital both to understanding the spread of the epidemic and to effective prevention efforts. Project narrative To be able to implement effective interventions a greater understanding of the complex multi-level nature of the HIV epidemic is necessary. The proposed research will lead to a greater understanding of causal pathways of HIV infection by identifying and quantifying important environmental, community, household and individual-level determinants of HIV infection. This information can be used to guide effective prevention efforts.