Adolescence is an important period of physical, social, psychological, and cognitive growth. No longer children and not yet adults, adolescents make significant choices about their health and develop attitudes and health behaviors that continue into adulthood. Substance use disorders among adolescents can impede the attainment of important developmental milestones, including the development of autonomy, the formation of intimate interpersonal relationships, and general integration into adult society. Similarly, the use of alcohol and illicit substances by youth often leads to adverse health outcomes. This paper presents: (1) facts about adolescent substance use; (2) factors that make monitoring and decreasing adolescent substance use especially difficult; and (3) recommendations for preventing adolescent substance use. (Contains 2 figures and 26 endnotes.)