This chapter presents an overview of prolonged exposure therapy (PE), a highly efficacious and effective treatment for post-traumatic stress disorder (PTSD). First, emotional processing theory is reviewed, which provides the theoretical basis for PE and the key mechanisms underlying PTSD symptom reduction. Next, a synthesis of the robust evidence for the efficacy and effectiveness of PE is provided. The chapter reviews evidence that in addition to ameliorating PTSD symptoms, PE reduces secondary symptoms such as depression, suicidal ideation, anger, and substance use disorders. The chapter describes evidence supporting the extension of PE with unique samples, including individuals with psychosis, persons with self-injurious behavior, and war veterans. The chapter concludes with a review of the status of PE dissemination and implementation efforts.