PROJECT SUMMARY. This direct to Phase II SBIR proposal will study Appa Health, an adolescent-facing mental health smartphone and computer app. Appa combines (1) delivery of short-form digital evidence-based mental health content, emphasizing Cognitive Behavioral Therapy, via educational videos delivered by mental health experts who are internet influencers, with (2) trained and supervised near-peer mentors with relatable lived experiences who use supportive accountability to facilitate adolescents’ use of the evidence-based content. According to Appa’s theory of change, near-peer mentoring directly impacts adolescents’ mental health and well-being via relational and instrumental support, but also indirectly through the acquisition of evidence-based skills (e.g., cognitive restructuring, behavioral activation) taught via educational videos. We will examine the psychosocial mechanisms of the two primary elements of Appa, establish the effectiveness of Appa on adolescent depression and anxiety, other measures of functioning, and theoretical mechanisms, and examine the cost-effectiveness of the full Appa experience as well as a video-only condition that Appa is considering marketing. Aim 1 will collect initial data and improve study procedures via a small pilot study. Seventy-five adolescents aged 13-18 with elevated scores on measures of depression and/or anxiety will be randomly assigned to one of three conditions: (1) near-peer mentoring and digital tools (Full Appa), (2) digital content only (Appa Light), and (3) waitlist control. Results will be used to optimize methodology to achieve study goals with minimum burden and maximum retention. Aims 2 & 3 will examine Appa’s cost and effectiveness via a well-powered randomized clinical trial replicating the pilot trial using 400 adolescents. Variables will include clinical outcomes such as adolescent symptoms of depression, anxiety, and well-being, as well as treatment mechanisms and proximal outcomes. Because of the mental health workforce shortage, this study will have an impact on science by demonstrating the effectiveness of using laypersons (near-peer mentors) combined with expert digital content delivery and clinical supervision to support adolescents’ mental health. This study is innovative in the unique combination of near-peer mentoring with evidence-based content delivered via youth-targeted TikTok-style videos conducted by existing internet influencers. Questions on how to increase adolescent engagement with and use of evidence-based skills are important to intervention developers. The data from this study will directly impact Appa’s commercialization plan. Appa is scaling quickly, expanding from direct-to-consumer sales to partnerships with schools and self-funded employers, both of which are eager for documented clinical outcomes and identification of replicable mechanisms of change to ensure quality as Appa scales services. Cost-effectiveness analyses will drive development and design de...