The development of the UK's military policy includes the potential for military organisations to deploy in support of humanitarian aid operations. This paper offers an overview of the risks to people's mental health of their exposure to emergencies, major incidents, disasters, terrorism, displacement, postconflict environments in which humanitarian aid is delivered, and deployments to conflict zones. It summarises the psychosocial approach recommended by many contemporary researchers and practitioners. It differentiates the extremely common experience of distress from the mental disorders that people who are affected may develop and introduces the construct of psychosocial resilience. The authors recognise the importance of trajectories of response in separating people who are distressed and require psychosocial care from those who require mental healthcare. Finally, this paper summarises a strategic approach to designing, planning and providing psychosocial and mental healthcare, provides a model of care and outlines the principles for early psychosocial interventions that do not require training in mental healthcare to deliver them.