The goal of this clinical trial is to learn if receiving Prolonged Exposure Therapy for PTSD in massed format (multiple sessions weekly) is as effective as receiving it with sessions once per week among veterans with PTSD and substance use disorder in intensive outpatient substance use treatment. The main questions it aims to answer are: * Will the massed format help participants complete and benefit from Prolonged Exposure in terms of PTSD symptoms? * Will it help participants reduce substance use? Participants who are in intensive substance use treatment will be asked to complete Prolonged Exposure with either weekly sessions or multiple sessions per week.