Depression in young people is a serious problem that can lead to lifelong poor mental health and stigma. Depression is reported in around 20% of under 18s, and over half continue to be depressed into adulthood. Problems include difficulties at home and school, maintaining friendships and taking part in social activities, including exercise. Young people with depression often delay seeking psychological support. Antidepressants can help, but they have negative side effects. Research shows that adults with depression benefit from exercise, but it is not known whether exercise is helpful for young people who are depressed. The aim of this study is to find out whether exercise is an effective treatment for young people with depression and whether it is good value for money for the NHS.