Difficulty controlling anger is the most commonly reported reintegration concern among combat Veterans, especially those with a diagnosis of posttraumatic stress disorder (PTSD). In Veterans, problematic anger is associated with numerous negative psychosocial outcomes, including poor functional outcomes (both social and occupational), family discord, aggression, road rage, and suicide risk. Anger can also impede successful outcomes from PTSD treatment. Given the high prevalence of anger problems among Veterans with PTSD and the associated functional impairments, there is a clear need to develop innovative and effective anger interventions to improve functional outcomes. Use of mobile health (mHealth) technology could provide a low-cost method to increase the reach of anger management treatments to this high-need group of Veterans. One of the mechanisms associated with problematic anger and aggression is hostile interpretation bias, i.e., a tendency to interpret ambiguous interpersonal situations as hostile. By reducing hostile interpretation bias, we can reduce problematic anger and aggression and improve functional outcomes. Our team has developed and piloted a mobile application that uses evidence-based interpretation bias modification techniques to reduce hostile interpretation bias and anger outcomes. The goal of this project is to evaluate the efficacy of this mobile application among Veterans with PTSD. A randomized controlled trial (RCT) is proposed, in which 150 Veterans with PTSD and problematic anger will be randomized to either the Mobile Anger Reduction Intervention (MARI) or a health education condition (HED). The central hypothesis is that participants in the MARI condition will experience greater anger reductions and functional improvements relative to the HED condition. The proposed research project will address the following Specific Aims: Aim 1: To evaluate the efficacy of MARI on anger outcomes (e.g., hostile interpretation bias, problematic anger) measured at post-treatment, 3-months, and 6-months post treatment; Aim 2: To evaluate the efficacy of MARI on functional improvements (i.e., psychosocial functioning, quality of life) and self-harm measured at post-treatment, 3-months, and 6-months post treatment; and Exploratory Aim: To explore whether changes in hostile interpretation bias (proposed treatment mechanism) mediate functional improvements. If shown efficacious, the public health impact of the MARI application could be enormous. MARI would provide clinicians and Veterans with an easily accessible treatment option to improve psychosocial functioning and anger outcomes for Veterans with PTSD. This low-cost, easily implementable, intervention would significantly increase the reach of evidenced-based treatment for anger.