Post-traumatic stress disorder (PTSD) is a serious disorder among Veterans and affects up to 20 % of Veterans from the recent conflicts. Given the high incidence of PTSD among Veterans, the US Department of Veterans Affairs (VA) and Department of Defense (DoD) have made the treatment of PTSD a priority. Cognitive Processing Therapy (CPT, CPT-C) is one of the standard evidence based therapies for PTSD and was rolled out nationally in a large dissemination project by the VA and the DoD as one of the gold standard treatments for PTSD. Recently the VA DoD Clinical Practice Guidelines on the Management of PTSD and Acute Stress Reaction (2017) have stated that individual trauma focused therapies should be the first line of treatment. Despite the effort in rolling out evidence based therapy, there is a lack of knowledge of the efficacy of these interventions in veterans with comorbid disorders. There is considerable evidence showing that PTSD is often comorbid with other substance use disorders including opioid dependence. Opioid use disorder (OUD) is a well-documented epidemic among the general population, and also has had a significant effect on Veterans. While the etiology is unknown, it is possible that the incidence may be in part due to the overuse of highly addictive prescription opioids leading to iatrogenic opioid dependence and the development of addictive disorders. Recent evidence has clearly shown that prescription OUD can lead to dependence on street drugs such as heroin. Buprenorphine treatment is the established treatment for those with opiate use disorders; it is the recommended treatment for Veterans with OUD and its use is leading to larger number of Veterans maintained on buprenorphine. However, the efficacy of treatments for PTSD among those maintained on buprenorphine is unknown. The objective of this study is to test a standard psychotherapy for PTSD in Veterans who also suffer from OUD. Specifically, this study will test whether Cognitive Processing Therapy (CPT)-C is more effective in treating PTSD, compared to a control group (Individual Drug Counseling or IDC; which approximates treatment as usual), among Veterans with PTSD and comorbid OUD who are maintained on buprenorphine. Other objectives include effect on opiate use, treatment retention, side effects, pain tolerance and general functioning. This will be a randomized, open-label clinical trial. The study has three phases. In Phase one, the induction phase, all Veterans (n=160) will be started on 2 mg of buprenorphine. The dose of buprenorphine will be increased over 5-7 days; dose will be clinically determined. After the maintenance dose of buprenorphine is reached all Veterans will enter Phase two, the treatment phase. During this phase Veterans will be randomly assigned to CPT-C or standard IDC for 12 weeks. They will be seen weekly for psychotherapy and also regularly (weekly, then biweekly, then monthly) for buprenorphine management, symptom evaluation, and medic...