Laboratory Studies Characterize resistance risk from clinical trials and demonstration studies to understand the duration of time an infected person can be on product before resistance is selected. Modeling Use mathematical modeling to identify the most effective and efficient HIV testing and resistance monitoring strategies during PrEP roll-out. Policy Developement Generate policy recommendations for HIV diagnostic testing frequency and ARV resistance monitoring based on laboratory and modeling data. Monitor Resistance Monitor seroconverters from PrEP roll-out programs for ARV resistance in selected clinics in South Africa, Zimbabwe and Kenya.