The aim of this book is to provide readers with a comprehensive introduction to the key elements of a new approach to youth mental health that has transformed global understanding of the topic and resulted in the redesign of mental health services for adolescents and young adults in many parts of the world. The book is structured around the key elements of this approach, providing an account of the developmental context of adolescence and early adulthood, the epidemiology of mental disorders in this age group, the challenges of mental health stigma, and involving young people in service design. The book will also include examples of new model services in Ireland and the UK. Eilis Hennessey is Professor of Developmental Psychology at University College Dublin. Caroline Heary is a Doctor in Health Psychology at NUI Galway