This chapter defines posttraumatic stress disorder (PTSD) according to the Diagnostic and Statistical Manual of Mental Disorders (DSM). The manual’s fifth edition (DSM-5) organizes PTSD symptoms into four clusters: intrusion, avoidance, negative alterations in cognition and mood and alterations in arousal and reactivity. It emphasizes that diagnosis requires symptom persistence beyond 1 month with significant functional impairment. Each of these clusters is evident in the experiences of the Australian veteran described in Chap. 1 . The PTSD checklist for DSM-5 (PCL-5) and the clinician-administered PTSD scale for DSM-5 (CAPS-5) are then compared to highlight their complementary roles for screening and clinical determinations. The 30%–50% comorbidity of PTSD with major depressive disorder (MDD) is then addressed, including their joint trajectory over a decade. Finally, regression and correlation analyses are used to evaluate the extent to which trauma as well as psychological and genetic factors predicts PTSD.