There are high rates of trauma exposure and posttraumatic stress disorder (PTSD) among treatment- seeking veterans with substance use disorders (SUD). Comorbid PTSD among veterans with SUD contributes to poor treatment outcomes and is associated with worse overall mental and physical health. While addiction programs traditionally do not address PTSD, increasing evidence suggests that trauma treatments not only treat PTSD, but may even improve SUD treatment retention and outcomes among those with comorbidity. Veterans with SUD are usually treated in SUD specialty clinics, where staff are not trained in trauma, and thus do not offer PTSD treatment. So, although the Department of Veterans Affairs (VA) prioritizes the administration of effective evidence-based psychotherapies for PTSD, only a small percentage of veterans with SUD receive this treatment. Written exposure therapy (WET) is a brief trauma-focused intervention shown to effectively treat PTSD. WET has several advantages over traditional therapies; it can be delivered in 5 sessions, has lower dropout compared to other PTSD treatments, has high patient satisfaction, and can be easily delivered with minimal therapist training requirements making it cost efficient and therefore an ideal intervention for administration in a busy outpatient SUD clinic. A previous study suggested written exposure is feasible and effective in decreasing symptoms of PTSD among women in residential SUD treatment. In a recent acceptability and feasibility pilot study, SUD treatment-seeking veterans (N=12) with comorbid PTSD were randomized to WET plus treatment as usual (TAU) or TAU augmented by a neutral writing condition. Results showed this writing-based intervention was feasible and acceptable. WET was associated with a greater decrease in PTSD symptoms at 8-week post-baseline follow-up and a greater decrease in the average number of days of alcohol and drug use at follow-up compared to controls. The proposed study is a randomized clinical trial within a SUD specialty clinic to evaluate whether TAU plus WET is superior to TAU augmented by neutral topic writing on both PTSD and addiction outcomes for veterans in SUD treatment. The proposed study will also examine a biological marker of autonomic arousal, heart rate variability (HRV), in order to determine whether it might be a potential objective measure of treatment efficacy. Emerging evidence suggests high-frequency HRV may be sensitive to SUD and PTSD symptom improvement and a possible mechanism underlying the connection between stress and substance use. [This study will examine the association between HRV and treatment outcomes. We will also examine whether change in HRV scores over time correlates with treatment outcomes (PTSD symptoms and days of substance use).] The specific aims of the project are to examine whether WET augmentation of TAU: a) improves trauma symptoms for veterans with SUD and PTSD who are receiving outpatient SUD treatment compared to T...