Military psychology is a specialist discipline within applied psychology. It entails the application of psychological science to military operations, systems and personnel. The specialty was formally founded during World War I in the UK and the USA, and it was integral to many early concepts and interventions for psychological and neuropsychological trauma. It also established a fundamental basis for the psychological assessment and selection of military personnel. During and after World War II, military psychology continued to make significant contributions to aviation psychology, cognitive testing, rehabilitation psychology and many models of psychotherapy. Military psychology now consists of several subspecialties, including clinical, research and occupational psychology, with the latter often referred to in the USA as industrial/organisational psychology. This article will provide an overview of the origins, history and current composition of military psychology in the UK, with select international illustrations also being offered., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2019. No commercial re-use. See rights and permissions. Published by BMJ.)