Project Summary/Abstract Experiencing mental health problems during adolescence is associated with symptom worsening and recurrence later in life, as well as poorer physical health and increased risk for preventable chronic diseases. Mindfulness-based intervention (MBI) has been shown to support key skills and capacities that may protect against mental health challenges, including emotion regulation (ER) and mindfulness. Yet, MBI effects on health and wellbeing outcomes in adolescents are generally small and inconsistent across studies. Theory and empirical evidence suggest that engagement, a multidimensional construct referring to the degree of productive involvement with an activity, may be a key change process underlying the effect of MBI on target intervention processes (i.e., ER and mindfulness). The proposed study will be the first to rigorously test engagement as a mechanism of MBI in adolescents. The specific aims are to (1) Test longitudinal and time-ordered change associations for engagement and target processes (ER and mindfulness) and (2) Utilize an embedded, sequential mixed-methods design to identify factors that facilitate and impede adolescent engagement in MBI. These aims will be tested in N=75 community adolescents participating in a single-arm trial of MBI on mental health and stress physiology. The current proposal is a novel extension of the parent study that incorporates longitudinal structural equation modeling and mixed methods to conduct a mechanistic examination of adolescent engagement in MBI. The proposal is designed to advance the candidate’s long-term career goal of leading research that deepens our understanding of the mechanisms of MBI and strengthens the science of its implementation, ultimately through supporting communities in effectively implementing and evaluating MBI for themselves. The training objectives are to: a) Develop the skills to analyze repeated measures to rigorously test engagement as a mechanism of MBI, b) Gain training in the integration of qualitative and quantitative data to more deeply understand MBI engagement, and c) Develop expertise in designing, implementing, and sustaining community-based research on mindfulness and adolescent health. The findings generated through this work will provide preliminary insights into the role of engagement in MBI efficacy for adolescents, an important step toward identifying pragmatic directions to optimize MBI for the prevention of adolescent mental health challenges. The training and development activities will enable the candidate to launch her career as an independent scientist, capable of effectively collaborating with communities to conduct research on MBI for the prevention of adolescent health challenges and the promotion of holistic wellness.