This study is being done to evaluate the relationship between adherence to antiretroviral therapy (ART) and HIV drug concentrations in persons living with HIV (PLWH) that are taking tenofovir alafenamide (TAF). Adherence will be measured with an ingestible biosensor (digital pill). Antiretroviral drug concentrations will be measured in different types of blood cells.

PLWH taking or initiating/re-initiating TAF will be recruited for a 16-week study. Upon entering the study, participants will be trained on the use the digital pill to objectively quantify adherence. Study participants already taking TAF will return every 4 weeks for a study visit where blood for drug concentrations will be obtained. Participants initiating/re-initiating TAF will return weekly for the first 4 weeks, at weeks 6 and 8, and every 4 weeks thereafter for a study visit where drug concentrations will be obtained.

Inclusion Criteria:

1. Females or males with HIV, able to give informed consent and comply with study procedures.
2. Currently on (> 6 months), or planning to initiate/re-initiate TAF.

Exclusion Criteria:

1. For females of childbearing age, active pregnancy or any intent to become pregnant
2. Hepatitis C (HCV) infection for which treatment is anticipated within the next 16 weeks after enrollment. Treated HCV with sustained virologic response is allowable if last dose of HCV antivirals was >12 months prior to enrollment
3. Advanced renal (eGFR <30 mL/min/1.73m**2) or liver (Child-Pugh B or C) disease
4. History of extensive bowel surgery, gastric bypass, or gastroparesis
5. Concomitant use of any prescription or non-prescription drug known (or with the potential) to significantly influence the PK of TAF (e.g. rifamycins, carbamazepine, phenytoin, St. John's Wort, tipranavir)

PLWH who are initiating/re-initiating or have been on TAF for > 6 months.