Thousands of mental health mobile applications (apps) are available but limited research has been conducted on their effectiveness. VA has been a leader in mental health mobile app development and must research whether these apps work, and if so, how? PE Coach is a well-designed treatment companion app to one of the most researched, efficacious psychotherapies for PTSD (prolonged exposure), a treatment that has been broadly disseminated throughout VA mental health clinics. Research suggests that VA therapists find the app helpful in supporting patients. Preliminary results suggest that Veteran patients prefer to receive therapy withPE Coach and Veterans complete more recovery-oriented homework when they do. This study will randomize 124 Veterans with PTSD to treatment with or without PE Coach. The project will evaluate the effect of the app on PTSD-related functioning, quality of life, and PTSD symptoms. The investigators will test whether the app improves functioning and symptoms, increases homework, and reduces drop out.