Psychological mechanisms can be defined as processes or events that are responsible for specific changes in psychological outcomes. In psychotherapy research, mechanisms are the factors through which interventions produce change. In this article, we explain the importance of identifying psychological mechanisms, describe methods for identifying them, and analyze recent literature on the psychological mechanisms underlying the development and treatment of posttraumatic stress disorder (PTSD). Based on the findings of recent investigations (from 2013 to present), we focus on four putative mechanisms: emotional engagement, extinction and contextualization, distress tolerance, and negative posttraumatic cognitions. Future directions for psychological mechanism research are also outlined, including possible opportunities for capitalizing on the most promising mechanisms identified to date.