Evidence suggests that in many countries, the mental health of young people has been declining over the past two decades, signalling a warning that global megatrends and changes in many societies are increasing mental ill health. Mental illnesses have their peak onset at age 15 years, and early intervention is crucial. The developing field of youth mental health focuses on individuals aged 12–25 years, viewing this period of emerging adulthood as more useful than dividing care at age 18 years. Mental health care for young people must be sensitive to this age group’s specific biological, cognitive, social, and cultural changes. This Commission considers the principles, core features, and strategies for designing, testing, and scaling up new models of youth mental health care, as well as promotion of mental health and prevention of ill health. It assesses the economic and political imperatives behind reforms in promotion, prevention, and care.