Koss CA, et al. – Researchers in this work sought to assess the optimal metrics of HIV preexposure prophylaxis (PrEP) adherence among adolescents and young men who have sex with men in the United States. Observations revealed that adherence waned over time among youth on PrEP. Self–report overestimated adherence and use of Wisepill devices was limited. Hair collection was highly acceptable and provided similar interpretations to dried blood spot (DBS); incorporating either metric in future PrEP studies among youth could identify sub–optimal adherence and trigger interventions to bolster adherence.