There is a large and growing body of evidence on the health benefits of engagement in leisure activities (voluntary, enjoyable non-work activities, such as hobbies, arts, volunteering, community group membership, sports, and socialising). However, there is no unifying framework explaining how leisure activities affect health: what the mechanisms of action are by which engagement with leisure activities leads to the prevention, management, or treatment of mental and physical illness. In this Review, we identify and map over 600 mechanisms of action.