Globally, concern is growing about the high and steadily increasing rates of anxiety and depression in young people. Effective and scalable ways of preventing poor mental health in this group are needed, and digital tools such as mobile apps have been proposed as part of the solution. Whilst there is emerging evidence for mental health apps being effective in treating anxiety and depression, this project led by the University of Exeter is the first to rigorously test a mental health app on such a large scale across four countries.