ABSTRACT HIV incidence remains high among youth aged 14-24 in Sub-Saharan Africa, thus underscoring the importance of interventions to prevent HIV in this age group. Nigerian youth are at the epicenter of an expanding HIV crisis, with the second largest number of new youth HIV infections of any country. Yet fewer than one in five Nigerian youth have ever been HIV tested. Young people are often framed as the fraught victims of new HIV infections, instead of recognizing their resilience and power in advancing effective response to the epidemic. Conventional expert-driven, top-down interventions have largely failed. Innovative solutions that leverage the resourcefulness and resilience of youth are sorely needed. In this proposal, we will develop innovative youth-friendly HIV self-testing services based on open challenges and apprenticeships that are informed by a youth participatory action research framework. Open challenges solicit solutions from the community and then implement the best solutions, while apprenticeship builds local capacity for youth-led projects to increase the likelihood of success and ensure comprehensive evaluation. Three randomized controlled trials based on open challenges have led to improved rates of HIV testing and condom use among at-risk youth, but none of these approaches have been in Africa. These participatory approaches will be used to identify innovative HIV self-testing strategies that meet the specific needs of Nigerian youth. Each strategy will have a comprehensive plan for enhancing HIV prevention (including HIV pre- exposure prophylaxis and behavioral approaches) among at-risk youth. Two HIV self-testing strategies will be selected by judges as semi-finalists and evaluated in separate local government area pilots (UG3 phase). Once the project meets stringent requirements demonstrating trial readiness and pilot effectiveness, a single strategy will be evaluated in a pragmatic, cluster randomized controlled trial in 24 local government areas of Nigeria (UH3 phase). The intervention development process will build capacity for decentralized HIV self- testing, drawing on the strengths, creativity, and engagement of Nigerian youth. Leveraging our extensive experience organizing HIV research in Nigeria, we propose the following specific aims: (1) To use open challenges and apprenticeship to develop new HIV self-testing services; (2) To evaluate the effectiveness of two semi-finalist participatory interventions on HIV testing and other key prevention services among at-risk, HIV-negative youth (14-24years old); (3) To determine the effectiveness of a finalist participatory intervention on HIV testing and other key HIV prevention outcomes in 24 areas using a stepped wedge, pragmatic randomized controlled trial. The overarching goal is not to create a monolithic ?one-size fits all? HIV self-testing strategy, but rather to demonstrate that open challenges can engage and inspire youth to develop innovative and effective HIV services. Our focus on open challenges and tailoring services to youth resonates with NICHD, NIH, and US government strategic priorities.