Veterans are at increased risk of posttraumatic stress disorder (PTSD), which negatively impacts social and occupational functioning and quality of life. Prolonged exposure (PE) is an evidence-based psychotherapy for PTSD that is effective in the treatment of Veterans. However, many Veterans drop out of psychotherapy for PTSD and many of those who complete PE still meet criteria for PTSD or do not achieve clinically meaningful change. The vast majority of Veterans with PTSD report neurobehavioral cognitive symptoms and may be at particular risk given that problems with executive functioning and prospective memory are related to PTSD treatment drop out and poorer treatment outcomes. The mechanisms of functioning and symptom change in PE include emotional activation during exposure, between-session habituation, and changes in PTSD-related negative cognitions. These mechanisms occur via verbally recounting and processing trauma memories (imaginal exposure) and approaching anxiety provoking situations in day-to-day life (in vivo exposure) and are achieved by daily homework exercises. Increased homework adherence predicts improved PTSD symptoms and functioning. To support patients participating in PE, the DoD/VA developed the mobile application (app) 'PE Coach'. The features of PE Coach were designed to enhance patients' ability to adhere with the homework critical to recovery. PE Coach may provide the cognitive and organizational support that Veterans with PTSD need to successfully complete treatment and maximize outcomes. However, little is known about the effectiveness of most mental health technology augmentations and this is true of PE Coach as well. Although the app is used in routine VA care and the VA recently invested in the software development required for an updated version of the app, no study has evaluated the impact of PE Coach on clinically relevant treatment outcomes. This study will conduct a randomized, controlled clinical trial to determine the efficacy of PE Coach and its impact on PTSD-related social and occupational functioning, PTSD symptoms, and homework adherence and dropout. The project will randomize 124 Veterans with PTSD to either PE +PE Coach or Usual PE (without PE Coach). Participants in both conditions will have identical homework activities assigned, but those with PE + PE Coach will receive all of their homework activities through the PE Coach app, while those in Usual PE will receive their homework through separate, standard workbook handouts and digital audio recording device. Veterans will participate in 8-15 weekly individual PE psychotherapy sessions, based on defined termination criteria and delivered consistent with the published treatment manual. Outcome assessments will be conducted at baseline, half-way through treatment, post-treatment, and at 1- and 4- month follow-ups. The primary outcome measure will be the Posttraumatic Stress Related Functioning Inventory (PRFI). PTSD symptoms will be assessed using the gold-standard Clinician Administered PTSD Scale for DSM-5, which will be conducted by independent assessors blind to treatment condition. Functioning, quality of life, self-reported PTSD, and depression will be monitored every-other PE session, throughout treatment. An external PE expert will code 15% of PE sessions to monitor the fidelity of psychotherapy to the treatment protocol, and homework adherence will be reported at each session to test the impact of PE Coach on adherence. VA has been a leader in mental health mobile app development and must respond to the call for research on whether these apps work, and if so, how? This study addresses that need by testing the efficacy of PE Coach, a treatment companion to one of the most researched, efficacious psychotherapies for PTSD, and a treatment that has been broadly disseminated throughout VA mental health clinics. Results can help inform whether VA should invest in systematic implementation and dissemination of PE Coach as a routine part of PE training in VA.