PROJECT ABSTRACT/SUMMARY Law enforcement officers (LEOs) are exposed to significant stressors, elevating their risk for aggression and excessive use of force. Such dysfunctional stress reactivity can lead to devastating consequences for their community, including unjustified shootings, severe beatings, and fatal chokings. It can also lead to serious consequences for the LEOs, including elevated incidence of post-traumatic stress disorder, burnout, alcohol misuse, depression, and suicide. A recent meta-analysis of LEO stress reduction programs found little evidence to demonstrate that current LEO stress management approaches are effective, highlighting the urgent need for interventions to successfully assist LEOs navigate the stress inherent to policing. One promising intervention is mindfulness training, which has been shown effective in reducing stress and increasing resilience with other high-stress populations. Our recent pilot feasibility study (R21AT008854) established initial feasibility, acceptability, and adherence to procedures in a single-site RCT assessing Mindfulness-Based Resilience Training (MBRT), a preventive intervention designed to improve LEO mental health and resilience, and reduce aggression and excessive use of force. The R21 data suggest physiological mechanisms and potential clinical benefit in a sample of LEOs. Relative to waitlist control, MBRT participants showed improvements in physiological indices of stress reactivity, aggression, burnout, resilience, and alcohol use. To prepare MBRT for a future multisite efficacy trial, the proposed study will refine procedures to: a) enhance efficiency of recruitment, engagement and retention; b) optimize lab and assessment procedures; c) optimize intervention training and ensure fidelity to intervention protocols; and d) ensure quality and fidelity of data management and sensitivity to change for all outcome measures. Implemented at two sites, this study will establish optimal protocols and procedures for a full-scale, multi-site trial assessing effects of MBRT versus an attention control (Stress Management Education) and a no intervention control, on physiological, behavioral, and psychological outcomes.