It is now known that the onset of severe and recurring mental health problems begins for the most part before the age of 25: this provides a clear focus for preventive strategies and public mental health that are a feature of many health policy frameworks. The present distinction between child and adolescent mental health services and adult services at 16 or 18 does not fit easily with these data and the now well-documented problems of transition suggest that a fundamental review of services for young people is overdue. This supplement provides an overview of the epidemiological, conceptual and service structures for young people with emergent and existing mental health problems, and asks the question, 'How should we design services for young people to promote prevention and service engagement, and to improve outcomes?'