Project Summary Posttraumatic stress disorder (PTSD) and opioid use disorder (OUD) co-occur at very high rates (33-66%). Untreated PTSD among people with OUD is associated with elevated risk of overdose, suicidality, and other serious outcomes. Exposure and other evidence-based treatments for PTSD are effective for patients with substance use disorders (SUD), including OUD, but rarely implemented in residential SUD treatment, representing a crucial missed opportunity to address PTSD symptoms. Written Exposure Therapy (WET) is a brief, evidence- based intervention for PTSD that shows promise in residential OUD/SUD treatment settings, but our pilot work has indicated a need for additional refinements to adapt WET for this setting. The proposed multisite study will build on the investigative team’s prior research projects that have explored WET for PTSD in the context of residential SUD treatment. We will use the 8-step ADAPT-ITT structure to create and test Written Exposure in Substance Treatment (WEST), an adapted version of WET for use with people with OUD in residential SUD treatment. We will obtain candid feedback from key stakeholders on the PTSD-related treatment needs of people in residential OUD/SUD care and the components, implementation, and candidate modifications of WET for residential OUD/SUD treatment, develop an initial adaptation of WET, obtain expert feedback from the developer of WET on the adapted version, and conduct an uncontrolled pilot test of the adapted version with N=20 participants. After this preliminary phase, we will examine the efficacy of the refined WET protocol in the context of residential OUD/SUD treatment by enrolling N=224 participants in a randomized controlled trial across two sites. In addition to the primary outcome of PTSD symptom reduction, we will test secondary and exploratory outcomes related to OUD/SUD outcomes, moderators and mediators of symptom reduction, and catalog facilitators and barriers of WET implementation in a residential OUD/SUD treatment setting. By expanding investigations of WET into residential OUD/SUD treatment settings, this project will significantly contribute to our knowledge base of practical strategies to provide comprehensive and integrated behavioral healthcare and reduce the impact of the current opioid epidemic.