Alcohol use disorder (AUD) and posttraumatic stress disorder (PTSD) frequently co-occur, and having both disorders is associated with greater psychological and functional impairment than having either disorder alone. The most effective PTSD treatment, prolonged exposure (PE) is sometimes less effective when individuals also have AUD. Anti-relapse medication appears promising to improve the effectiveness of PE to help individuals reduce alcohol use and PTSD symptoms and improve functioning. This study compares PE with and without topiramate, a medication shown to both reduce drinking and PTSD symptoms, with the hypothesis that combined PE and topiramate will be more effective than PE and placebo. The aim of this grant is to improve treatment outcomes for Veterans with AUD and PTSD.