This is an open access title available under the terms of a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 International license (CC BY-NC-ND 4.0). It is free to read at Oxford Academic and offered as a free PDF download from OUP and select open access locations. Sponsored by the Adolescent Mental Health Initiative of the Annenberg Public Policy Center (APPC) of the University of Pennsylvania and the Annenberg Foundation Trust at Sunnylands Trust, the third edition of Treating and Preventing Adolescent Mental Health Disorders provides a significant update since the second edition in 2017. It addresses the current state of knowledge about the major mental health disorders that emerge during adolescence, including how the COVID-19 pandemic affected adolescent mental health. Here, experts on adolescent anxiety, schizophrenia, substance use disorders, depression and bipolar disorders, eating disorders, and suicidal behavior thoroughly define each disorder, outline and assess available treatments, discuss prevention strategies, and suggest a research agenda based on what we know and don't yet know about these various conditions. The volume also incorporates chapters on gambling, Internet addiction, and positive youth development, which addresses how we can fully prepare young people to be happy and successful throughout their lives. Concluding chapters discuss overarching issues regarding the behavioral and mental health of adolescents, including overcoming the stigma of mental illness and the research, policy, and practice context for the delivery of evidence-based treatments. Integrating the work of eminent scholars in both psychology and psychiatry, this work will be an essential volume for academics and practicing clinicians and will serve as a wake-up call to mental health professionals and policy makers alike about the state of our nation's response to the needs of adolescents with mental disorders.