This paper describes the history of the development of understanding of psychological responses to traumatic life-events and their treatment. One major response, post-traumatic stress disorder (PTSD), is a recognized condition which has appeared relatively recently in diagnostic manuals. PTSD is a condition of major significance, not only to mental health professionals, but also to trauma surgeons and allied professionals. This paper focuses on the current definition of PTSD in the International Classification of Diseases (ICD-10, 1992, World Health Organization) and the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM-IV, 1994). PTSD first appeared as an operational diagnosis in DSM-III (1980) and was revised in DSM-III-R (1987) and DSM-IV (1994). It made its first appearance in the ICD system later, in 1992. This paper seeks to increase awareness of PTSD across the broad spectrum of trauma management professionals, to emphasize the practical value of identifying the disorder and to encourage optimism for its treatment. This paper is in two parts. The first part deals with historical development and classification. The second part (to appear in the next issue of Injury) deals with treatment.