Posttraumatic Stress Disorder (PTSD) and Substance Use Disorder (SUD) are highly comorbid, and comorbidity increases risk for poor functional outcomes. SUDs are associated with poor functional outcomes such as quality of life, community engagement, and suicide (Teeters, Lancaster, Brown & Back, 2017). Risks for poor quality of life and suicide increase further for those with co-occurring PTSD and SUD diagnoses as compared to either condition alone, with suicide attempt rates three times higher for Veterans with alcohol use disorder and PTSD (Norman, Haller, Hamblen, Southwick & Pietrzak, 2018). For patients with PTSD-SUD, there is evidence of greater PTSD symptom severity and poorer SUD treatment outcomes (e.g., Back et al., 2000), as well as higher rates of homelessness and disability (Bowe & Rosenheck, 2015). PTSD-SUD treatments have shown promising reductions in PTSD and SUD symptoms (Flanagan, Korte, Killeen & Back, 2016). Yet, there are still major challenges in widely implementing concurrent or single-target gold-standard treatments for this population, especially with rural veterans where care access may be limited (e.g., Flanagan et al., 2016). Written Exposure Therapy (WET) is a front-line, brief and effective treatment for PTSD that addresses some of the challenges posed by other gold-standard treatments. This project is designed to examine the feasibility and acceptability of Written Exposure Therapy (WET) delivered to Veterans with comorbid PTSD-SUD while they are completing a 28 day-residential SUD program (DOM SUD). The preliminary effects of the treatment during the program, and at one month and 3 month follow-up periods will also be examined, with particular attention to rates of substance use, homelessness, treatment attendance, treatment completion, quality of life, suicidality, and PTSD and depression symptoms. Veterans enrolled in the DOM SUD with PTSD Checklist-5 (PCL-5) scores over 33 will be recruited for further screening into the study. Those that meet criteria for PTSD through confirmation using the Clinician Administered PTSD Scale for DSM- 5 (CAPS-5) will be randomized into one of two treatment arms: Treatment as Usual (DOM SUD) and Written Exposure Therapy in a residential SUD program (resWET). Randomization will be stratified by gender and oversampling for women and minority participants will be used. Those in the TAU control group will participate in the DOM SUD treatment program, while those in the resWET group will also have five individual treatment sessions of WET. Participants will complete weekly measures of symptoms, in addition to rating cravings for substance use. Treatment completion rates will also be compiled for both DOM SUD and resWET. Participants will complete pre-treatment, post-treatment, 1 month, and 3 month follow-up measures in person, over video, or by phone, and these measures, along with data available from the treatment record, will be examined using descriptive analyses to look for importan...