Young people are arrested at high rates in the U.S., particularly in minoritized communities, and face stark behavioral health disparities related to substance use and sexual health compared with peers who have not been arrested. Low emotion regulation appears to be a common mechanism underpinning these disparities and it can be improved via mindfulness meditation. Moreover, meditation can be taught by smartphone app, holding the potential to reach youth on probation—who make up the large majority of all young people in the legal system—in their daily lives. In a prior NIDA-funded study (K99/R00DA047890), we collaborated closely with youth and other stakeholders in Chicago Cook County, the 2nd largest juvenile legal system in the U.S., to identify determinants of implementing a 1-month mindfulness meditation app with youth on probation, develop a package of corresponding implementation strategies to promote app adherence (i.e., ongoing usage), and run a fully- remote pilot RCT with n=70 youth on probation. The pilot data strongly supported feasibility and acceptability including very high objective app adherence (mean=20.40 of 30 daily meditations, SD=7.99) and retention at 1 (86%) and 6 (80%) months. There were also multiple signals of greater reductions in substance use problems and sexual risk behaviors among youth randomized to the meditation app than to a health promotion control app matched for time and structure. Throughout the pilot RCT, youth and officers requested that the meditation app be sustained in probation programming if it demonstrates effectiveness. This proposal is a response to our stakeholders’ requests. The proposed study will proceed in 2 main steps corresponding to (1) testing the effectiveness and individual-level implementation of the meditation app, and (2) starting to plan for its sustainability on the organizational level. First, we will leverage our successful pilot RCT design to run a hybrid type 2 effectiveness-implementation RCT with N=300 youth on probation in Cook County. Youth will be randomized to use the meditation or health control app daily for 30 days. Objective usage data will identify non- use of both apps and trigger adherence support (e.g., texts) via an adaptive intervention design. Youth will report on the health outcomes (substance use and sexual health) and the mechanistic target (emotion regulation) at baseline, 1, and 6 months. They will also complete 1-week “bursts” of ecological momentary assessment (EMA) at baseline and 1 month to supplement the retrospective emotion regulation data with real-time, ecologically valid reports. Together, this will allow us to test if the meditation app is effective in reducing substance use problems and sexual risk (Aim 1a), if improvements in emotion regulation mediate these effects (Aim 1b), and if youth adequately adhere to the app (Aim 2). To start systematically planning for sustainability of the meditation app in probation programming (Aim 3), we will int...