In contrast to all other age groups, HIV-related mortality has increased rather than decreased over the last decade among youth between the ages of 10 and 19 years. This is due in part to poor retention in care and adherence to antiretroviral therapy (ART) in this age group. As youth living with HIV enter adulthood, they transition from pediatric to adult HIV care. The transition to adult care presents heightened challenges to retention, due to disruption of established relationships with pediatric healthcare providers and a shift to an adult care model that requires greater autonomy and offers less specialized support. Resources to support youth through the transition are lacking. Mobile technology for health promotion (mHealth) using social media is a promising approach to maintain engagement in care through the transition. mHealth reminders, education, and support from healthcare workers have been successful in improving ART adherence in the adult HIV care setting and there have been limited, though promising, results in youth care. This planning grant will support the development of a novel mHealth strategy for the Pediatric to Adult HIV Care Transition (mPACT) in preparation for a future cluster-randomized trial in Kenya. The conceptual framework for this intervention is based on providing support to youth who are transitioning to adult care through a combination of virtual group peer support and 1-to-1 communication with a healthcare worker trained in youth HIV care. These modes of support will be facilitated by an mHealth platform that will enable a high degree of interaction and tailoring of communication content. The aims of this R34 Clinical Trial Planning Grant are to (1) identify the specific barriers to successful youth transition to adult HIV care, develop the mPACT intervention messaging strategy, and create a prototype of the mHealth platform; (2) Pilot the mPACT intervention to determine its effect on the intermediate outcomes of transition preparedness, ART knowledge, stigma, depression, social and caregiver support; (3) Prepare protocols, materials, and study sites for the future cluster-randomized trial. The growing ubiquity of mobile technology among youth presents an accessible and flexible medium through which to engage this vulnerable group, and maintain continuity of relationships during the period of transition. This planning grant will enable design of an intervention that is responsive to the concerns of youth and their healthcare providers, and prepare for a larger R01 cluster-randomized trial evaluating its clinical impact. Improving the transition from pediatric to adult care will result in greater adherence and reduced loss-to-follow- up, leading to improved health for youth living with HIV and reduced transmission in the population by increasing the number of youth with suppressed virus.