Although symptoms characteristic of post-traumatic stress disorder (PTSD) have been noted in military personnel for many centuries, it was not until 1980 that the disorder was formally recognized and became the focus of legitimate study. This paper reviews our current state of knowledge regarding the prevalence and course of this complex condition in past and present members of the defence forces. Although rates vary across conflicts and countries, there is no doubt that PTSD affects substantial numbers of personnel and results in considerable impairment in functioning and quality of life. The paper goes on to discuss recent attempts to build resilience and to promote adjustment following deployment, noting that there is little evidence at this stage upon which to draw firm conclusions. Finally, effective treatment for PTSD is reviewed, with particular reference to the challenges posed by this population in a treatment setting.