Adolescent mental health is defined as positive adaptive functioning that can lead to healthy growth and well-being in addition to the negative adaptive functioning that can lead to mental illness. Since many mental health problems emerge in adolescence, it is essential to understand what variables lead to healthy adjustment and well-being. The purpose of this entry is to provide a definition of adolescence; an overview of adolescent mental health; the current brain research that supports the connection between brain function, development, and behavior; the environmental challenges that impact healthy and unhealthy adjustment; an overview of adolescent mental disorders; healthy mental health habits; and prevention and treatment methods for the promotion of healthy functioning.