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IntroductionDaniel Romer and Elaine F. WalkerThe Annenberg Foundation Trust at Sunnylands, under the direction of KathleenHall Jamieson, sponsored the Adolescent Mental Health Initiative in 2003 culminatingin Treating and Preventing Adolescent Mental Health Disorders (Evanset al., 2005, Oxford University Press). This project synthesized the current stateof knowledge about the emergence and course of major mental disorders in adolescenceand what can be done to treat and prevent these illnesses. As part of thiseffort, it became clear that our knowledge of the brain and its development fromchildhood through adolescence has increased dramatically in the last decade andthat this greater understanding opened new opportunities to prevent the mentaldisorders that often emerge during adolescence.One of the most exciting prospects was the increasing realization that the brainremains highly plastic throughout development. However, during adolescencemajor forms of brain reorganization take place that make this period particularlysensitive to preventative interventions. Much of the reorganization and maturationthat appears to occur during this period has been characterized as a source ofmaladaptive behavior, such as heightened risk taking, rather than an opportunityfor growth. The extensive pruning, especially in the prefrontal cortex, that continuesthroughout adolescence suggested to scientists, as well as the press, thatthe adolescent brain was a work in progress that was not prepared for the debut of1