ABSTRACT More than one million firefighters in the United States provide critical emergency medical services in communities they serve and have been on the front lines of healthcare delivery, including throughout the COVID-19 pandemic. As a result of exposure to occupational stressors, a high proportion of firefighters experience considerable stress-related illness burden, including chronic pain, psychological distress (i.e., anxiety and depression), and posttraumatic stress disorder. Although interventions have been developed to address this high need of reducing effects of occupational stress exposure in order to improve firefighter well- being (e.g., cognitive-behavioral therapy, resilience training), not all of these modalities appeal to all firefighters, nor are they easily implemented without direct, in-person contact. One modality that has shown promise to reduce distress in various populations is meditation, including meditation delivered by smartphone apps. To the best of our knowledge, no smartphone-based meditation interventions designed to cultivate both mindfulness and feelings of social connection to others have been tested with firefighters. This study will therefore test the efficacy of a 10-day, smartphone-based meditation app intervention among N=192 career firefighters. The app was developed by Health Minds Innovations (HMI, Madison, WI) and is designed to enhance both mindfulness (awareness) and social connection to others in order to reduce anxiety. Our group recently piloted tested this app and found that firefighters exhibited reduced anxiety (a key component of psychological distress) and burnout, as well as improved function of the stress hormone, cortisol, from before to after use of the app. Although these encouraging results suggest a low-cost, scalable smartphone-based meditation app may be effective to improve firefighter well-being, our pilot study lacked an attention control comparison needed to establish intervention efficacy. Therefore, in collaboration with 3 metropolitan fire departments in the United States, we will test the efficacy of the HMI meditation app to reduce psychological distress compared to a rigorous active attention control (i.e., a Health Education app, based on our prior work)(Aim 1). We will also determine whether the effect of the HMI meditation app of psychological distress is mediated by mindfulness and perceived social connection (Aim 2). The proposed research will provide important evidence of efficacy about a smartphone-based meditation app intervention, with likely high impact, that cultivates both mindfulness and social connection in order to reduce psychological distress in frontline workers in the fire service. This research supports NIOSH's Strategic Goal 7 / Activity Goal 7.14.1 (to develop interventions that integrate protection from work-related health hazards with promotion of prevention to advance worker well-being) and NORA Objective 6 (promote healthy work design and well-be...