In the US, fewer than 6% of all youth living with HIV (YLWH) achieve HIV viral suppression. This health disparity extends to the entire HIV care continuum in that there is a strong association between younger age and later HIV diagnosis, lower engagement in care, lower levels of antiretroviral therapy (ART) adherence, and worse HIV clinical outcomes. In response to this critical public health dilemma and aligned with the National Institute of Health's High Priority Topics of Research (i.e., “to reduce health disparities in the incidence of new HIV infections or in treatment outcomes of those living with HIV/AIDS”), our research team proposes to develop a novel mobile health application (“app”) to improve engagement in health care and ART adherence and to pilot test this mobile health app in 18-29 year-old YLWH. The aims of our study are to: Aim 1: Build on our theory-guided model and formative work to complete the development of our novel personalized mobile health app, WYZ (pronounced “wise”), for improved HIV clinical outcomes among YLWH. Our research team will work with the University of California, San Francisco's Information Services Unit to complete the development of WYZ using an iterative Human-Centered Design approach. This will be done by adding data integration with pharmacy and laboratory databases, creation of an Android-compatible version of the app based on our in-progress iOS version, refinement based on user-experience data from field testing, and qualitative interviews with the primary care providers of the youth participating in the field testing. Aim 2: Conduct a pilot randomized trial to assess the feasibility, acceptability, and preliminary clinical impact (i.e., adherence, plasma HIV RNA, and CD4+ cell count) of the use of WYZ among YLWH and their HCPs. Aim 2 consists of two sub-aims: 2a. Examine feasibility and acceptability of the use of WYZ based on defined metrics, acceptability through qualitative exit interviews, and preliminary clinical impact by virologic/immunologic outcomes with participants randomized to the intervention versus the wait-list control. 2b. Assess the acceptability of WYZ by participants' HCPs through individual qualitative interviews. We hypothesize that our mobile health app will be feasible and acceptable and will result in improved HIV clinical outcomes. Our study is significant because it has the potential to improve engagement in care, ART adherence, and clinical outcomes among YLWH. Our study is innovative because we will employ a novel suite of technology-based methods to improve information, motivation, and behavioral skills in YLWH. Upon completion, we will be ready to test the efficacy of this app in a subsequent large-scale RCT among this population that is disproportionately impacted by HIV and at elevated risk for poor clinical outcomes.