Posttraumatic stress disorder (PTSD) affects people of all ages and backgrounds and causes persistent suffering and impaired function, but its diagnosis offers the opportunity for early intervention. It is the subject of intensive developmental, epidemiological, genetic/genomic, translational, neurobiological, neuropsychological, and psychological research, and emerging computational methods with “big data,” statistical modeling, and machine learning are likely to accelerate this research. The findings from research on PTSD are changing education and the ways clinicians practice, offering the hope for improved care of those experiencing traumatic stress. Those at particular risk for PTSD include children and adolescents, women, soldiers, refugees and survivors of genocide, sexual orientation minorities, racial and ethnic minorities, patients with burns, injuries and medical trauma, and victims of rape, violence, accidents, and disasters. This chapter provides an overview of PTSD, covering Diagnostic and Statistical Manual of Mental Disorders (fifth edition) diagnostic criteria, epidemiology, neurochemistry and neurobiology, biological and psychological models, assessment, and treatment.