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The understanding that antiretroviral treatment prevents HIV transmission through suppression of viral load provides significant new opportunities in HIV prevention. However, knowledge of efficacy is only a first step to having an impact on the spread of HIV at a population level, the ultimate goal of all primary prevention modalities. This review explores what we know about treatment as prevention and how it could be used as a tool, as part of a combination approach, in the global response to HIV.Efficacy data show that treatment as prevention works at high levels in trial conditions in stable serodiscordant couples; a finding that can reasonably be generalized to other populations at risk of transmitting the virus. Modelling shows that treatment as prevention should have an impact, but the extent of this depends primarily upon whether optimistic or pessimistic assumptions are made about the programmatic use of antiretrovirals (ARVs).We describe research questions that need to be addressed in developing optimal programmatic public health treatment strategies including how best to target and implement the use of treatment as prevention, how to balance the needs of treatment for the individual patients’ clinical benefit against population level benefits, and how to create programmes that are able to link people to and retain them in care.