This study tests whether the use of Mindfulness Based Relapse Prevention (MBRP) in emerging adult (EA) substance use treatment results in changes in physiological chronic stress measures and improved treatment outcomes. Further, this study will investigate the impact of Adverse Childhood Events (ACEs) on changes in chronic stress measures and treatment outcomes. While stress remains a robust predictor of substance use, individuals who experience ACEs are at increased risk of use and negative health outcomes. Treatments such as MBRP supply mindful meditation training (stress reduction) and relapse prevention skills to aid in improved treatment outcomes and decreased health and substance use problems after treatment discharge. By randomly assigning EAs to receive MBRP or treatment as usual this study will investigate if innovative treatments designed to reduce stress can improve treatment outcomes for a high risk and marginalized sample of EAs. In addition to testing main effects of MBRP on treatment outcomes, this study attempts to determine mechanisms of change through which MBRP operates. Specifically, this study examines whether treatment assignment impacts changes in physiological measures of chronic stress and if these changes in stress account for changes in substance use outcomes. Further, we will also test whether this process is moderated by experience of ACEs. That is, we will investigate if individuals who have experienced ACEs show attenuated response to main treatment effects and if these individuals also show differing patterns in chronic stress measures. By utilizing treatment sites that serve high risk and marginalized EAs we are able to address concerns raised by the Institute of Medicine on the treatment and focus on high risk EAs. This study is able to test novel hypotheses about whether MBRP treatment effects work through changes in chronic stress measures and if mechanisms of change work equally as well for individuals with more early childhood stress.