Major (also called clinical) depression is a mood disorder that causes persistent feelings of sadness and may interfere with work or school, eating, and the ability to enjoy life. Depression among adolescents impacts millions of young people and their families each year. When family members suspect that an adolescent has the symptoms of depression, assistance should be sought from a medical or mental health professional. The earlier an adolescent can receive assistance, the better. In addition, other issues may present with depression, such as anxiety, posttraumatic stress disorder, or substance abuse. This entry discusses the prevalence of depression in adolescents, diagnostic criteria, risk factors, developmental factors influencing depression, and prevention and intervention.