Left untreated, internalizing problems, such as a depressive or anxious mood, negative self-perceptions, and emotional distress, can undermine one's ability to succeed in school, live a healthy lifestyle, form and maintain close relationships with others, and, in general, accomplish life goals. When internalizing problems are experienced daily for at least two weeks, a psychiatric disorder--such as a major depressive disorder or a generalized anxiety disorder--may be underlying these problems. Among 13- to 18-year-olds, the lifetime prevalence of anxiety disorders is about 32 percent and the lifetime prevalence of mood disorders is about 14 percent. Among the population of U.S. adolescents aged 12 to 17, about 8 percent (about 2 million) had a major depressive episode (moderate to severe depression lasting at least two weeks) during the past year. Half of all adult lifetime cases of mental disorders emerge by age 14 years. Depression and other mood disorders increase risk for suicide, which is the third leading cause of death among U.S. adolescents aged 15 to 19. Overall, the cost of mental health problems has been estimated at about 2.5 percent of our Gross National Product, with $73.4 billion (1997 dollars) spent solely on the treatment of mental illness. This synthesis presents lessons learned from 37 random-assignment social intervention programs for adolescents that are designed to prevent or treat internalizing problems. Programs were identified by searching LINKS (Lifecourse Interventions to Nurture Kids Successfully), Child Trends' online database of rigorously-evaluated social interventions for children and youth. All interventions included in LINKS are social interventions evaluated using random assignment, intent-to-treat evaluations. Note that although this synthesis reviews treatment approaches, it does not review biomedical studies as these studies are not included in LINKS. Findings from this literature review suggest that social interventions to address internalizing problems are most effective when they teach adolescents how to cope with negative thoughts and emotions, solve problems, and interact effectively with others. Therapeutic approaches, such as family therapy, group therapy, individual therapy, and treatment-focused, school-based approaches appear to be effective. Mixed results were found for programs including activities to increase self-esteem and programs directed at non-clinical populations of youth. Among a handful of studies reviewed, programs with a mentoring component and programs targeting only females, although found to improve certain outcomes, were not found to ameliorate internalizing symptoms. (Contains 1 table, 10 footnotes, and 14 endnotes.) [This paper is a product of a co-partnership with the National Adolescent Health Information and Innovation Center (NAHIIC) at the University of California, San Francisco. Additional funding was provided by the Alexander and Margaret Stewart Trust Foundation.]