About 1.1 million firefighters (FFs; 350,000 professional or paid, 750,000 volunteers) protect Americans, their households, businesses, highways, and wildlands, performing emergency response despite significant risk to their own health and well-being. Even though FFs are screened for mental health risk upon entry into the profession, a substantial minority develop mental health problems during fire careers including posttraumatic stress disorder (PTSD), problematic alcohol use, depression, and anxiety. As might be expected with elevated rates of mental health conditions, suicide risk is a major concern among FFs. In sum, the occupational stress of fire service contributes to the early mortality of FFs who die 2 years earlier than other American males. It is imperative that FFs are equipped with more effective emotion regulation skills upon entry into the profession to buffer against the varied stressors that they will face. Waiting until mental health challenges emerge is a flawed approach. Barriers to mental health treatment seeking for FFs are numerous; FFs would benefit most from training in emotion regulation skills delivered by credible peers upon entry and socialization into the profession. This would likely prevent substantial negative outcomes downstream while promoting resilience and wellness. Training to prevent development of PTSD, alcohol misuse, and other commonly co-occurring mental health challenges should be transdiagnostic, focusing on the underlying mechanism of emotion regulation. The proposed project is the next logical step in our line of research whose long-range goal is to translate significant scientific findings to prevent the development of disabling mental health challenges in FFs. Our central hypothesis is that peer- delivered Brief Unified Protocol for Transdiagnostic Treatment of Emotional Disorders (BUP), delivered during the fire academy, will prevent the development of PTSD and related mental health outcomes. We will conduct a cluster randomized controlled trial (RCT) to compare peer- delivered BUP to peer-delivered psychoeducation regarding mental health (PSYED) to small groups of FF trainees during the fire academy for preventing symptoms of PTSD, alcohol misuse, depression, anxiety, and functional impairment over time in new FFs. We will test whether changes in neuroticism and emotion regulation mediate the effect of BUP on the outcomes. This would be the first RCT of a mental health prevention intervention in FF recruits, an important but understudied group with frequent trauma exposure and the first delivered by peers. If successful, this approach would provide a culturally competent, scalable approach to promoting resilience, thereby preventing work-related psychological injuries.