We reviewed epidemiological findings for the diagnosis of posttraumatic stress disorder (PTSD) and its core diagnostic features, focusing on whether epidemiology has been helpful in clarifying some of the critical diagnostic issues relevant to the revision of the Diagnostic and Statistical Manual of Mental Disorders and the International Statistical Classification of Diseases. Though epidemiology has provided increasingly rich data and knowledge regarding prevalence and incidence, patterns of onset and course, comorbidity, and risk factors for traumatic experiences and posttraumatic stress, little systematic research has been performed specifically addressing such critical diagnostic issues. Particularly, unresolved concerns remain regarding the definition of trauma, duration and impairment/distress criteria, the distinctiveness of the PTSD-syndrome, and even the position of PTSD in the classification system of mental disorders. A further exploitation of the existing data, and an improvement of existing epidemiological methods, strategies, and assessments are likely to substantially contribute to the clarification of unresolved diagnostic issues.