Mental health promotion and the prevention of mental health conditions, self-harm and risky behaviours are key to helping adolescents thrive. There is a growing evidence base on interventions to care for and treat mental health conditions. A range of multi-sectoral strategies and programs have been shown to have positive returns. However, more research is needed on this neglected area of adolescent health and well-being, to strengthen measurement, improve existing interventions and to design new scalable intervention packages for mental health promotion, prevention and care in adolescents. This knowledge summary calls for increased attention to and action on adolescent mental health, where everyone has a role to play.