Populations in sub-Saharan Africa are the most mobile in the world: up to 80% of households have at least one migrant member. Mobility is a risk factor for HIV transmission, but its impact on medication adherence for persons living with HIV (PLHIV) remains poorly understood. We will leverage the ongoing study Understanding Mobility and Risk in SEARCH Communities (RO1MH104132), which has followed over 1100 PLHIV from 12 East African communities since 2016, to study this association. Detailed measures of mobility, including migrations (changes of residence over geopolitical boundaries), geographies (origin-destination pairs), duration and frequency of travel, and reasons for mobility, are collected every 6 months, with hair samples collected annually. Adherence will be assessed using an objective measure, specifically antiretroviral concentrations in hair measured via liquid chromatography/tandem mass spectrometry. This RAP proposal, if funded, would allow for measurement of antiretroviral concentrations in hair, making this the first study to rigorously measure both mobility and adherence in a highly mobile cohort of PLHIV in Africa. Using this objective measurement of adherence, we propose to: (1) characterize the impact of mobility in East Africa on adherence to antiretroviral therapy among PLHIV across 4 years; and (2) estimate the association between hair antiretroviral concentrations and virologic suppression in this highly mobile community-based sample. We will evaluate associations between metrics of mobility and the objective adherence measure with multivariable mixed linear models, and the association between hair antiretroviral levels and virologic suppression with mixed-level logistic regression. This proposal will also a) help train this early career investigator on adherence metrics in preparation for a K-level application; b) inform the use of hair to determine adherence among mobile individuals in resource limited settings; and c) help determine whom to target for adherence interventions based on patterns of mobility and risk.