PROJECT SUMMARY Teacher skills are widely regarded as critical to the quality of mindfulness-based interventions (MBIs), but we lack data about which teacher skills matter in MBI. Without better definition of how to measure teacher skills and which skills influence participant outcomes, a central mechanism of MBI effects remains a ?black box.? Defining which teacher-related factors can be feasibly measured and shown to predict participant outcomes is important in selection of teachers for MBI research studies and monitoring of intervention delivery quality, and for strengthening teacher training for research and clinical programs. We aim to address this critical gap in MBI research through testing the planned outcome measures, the teacher rating scheme, and study procedures in preparation for a large-scale R01 study. In this R34, we plan to enroll 19 MBSR teachers and approximately 200 MBSR participants enrolled from the teachers' classes. Enrollment will be performed at two core sites (UCSF and UMass) and two community MBSR programs (in Los Angeles and New York). Community sites are included as we believe inclusion of a range of sites will be valuable in our planned RO1. Participants will complete questionnaires with patient reported outcome measures at baseline (before MBSR), 2 months (completion of MBSR) and 4 months. We will address the following aims. In aim 1, we will develop a standardized approach to training raters in use of the Mindfulness-Based Interventions Teaching Assessment Criteria (MBI:TAC), a recently developed instrument that uses expert raters to evaluate six domains of MBI teacher skills after viewing two hours of video recording of teaching. We will then test important characteristics of the MBI:TAC, including inter-rater reliability in our study setting. In aim 2, we will obtain stakeholder input into our participant outcome measure selection, refine measures if needed, and then assess feasibility, acceptability, and performance of the planned measures in our n=200 study to ensure appropriate measures for an R01. In aim 3, we will test strategies to maximize enrollment and retention of MBSR participants in the study to select procedures for a R01; this will include use of a factorial trial design to compare two different approaches to presenting the study in an introductory video, and two participant reimbursement approaches. These three aims will lay the groundwork for an R01 to test whether measurable teacher factors are associated with better participant outcomes. This sequence of planned studies promises to substantially advance our knowledge of how to measure teacher factors in MBIs and how these factors, in turn, are associated with participant outcomes. If successful, this line of research will open up a key ?black box? in MBI research and provide tools with broad applicability to a range of future research.