The COVID-19 pandemic has raised huge concerns for the mental health of an entire generation of children. But the pandemic may represent only the tip of a mental health iceberg – an iceberg we have ignored for far too long. For the first time, The State of the World's Children 2021 examines the mental health of children and adolescents. Against a backdrop of rising awareness of mental health issues, there is now a unique opportunity to promote good mental health for every child, protect vulnerable children, and care for children facing the greatest challenges. Making that happen will require urgent investment in child and adolescent mental health across sectors – not just in health – to support proven interventions. It will also need societies to break the silence surrounding mental health, by addressing stigma, promoting understanding, and taking seriously the experiences of children and young people.