Preventing and reducing acting-out or externalizing behaviors is a goal often targeted by out-of-school time programs for children and youth. The term "externalizing" refers to disruptive, harmful, or problem behaviors that are directed to persons and/or things. Examples include aggressive behaviors such as getting into fights and bullying; hyperactive and impulsive behaviors; and delinquent behaviors such as physical assault, theft, and vandalism. Children who engage in these behaviors are more likely than their peers to be rejected and bullied by peers and experience academic difficulties, and, as adolescents, they are more likely than their peers to engage in sexual-risk taking and substance use. Children and youth with externalizing problems are also more likely to engage in criminal behavior and to abuse substances in adulthood. Thus, integrating effective practices for preventing and reducing externalizing behavior into existing programs and services holds the potential to prevent a host of social, academic, and behavioral problems. This synthesis presents lessons learned from 123 intervention programs for children and youth that are designed to prevent and/or deter externalizing behavior. Programs were identified by searching LINKS (Lifecourse Interventions to Nurture Kids Successfully), Child Trends' online database of social interventions. All interventions selected for LINKS employ random assignment, experimental studies with intent-to-treat analyses. Because most programs include multiple intervention components, a single impact study is rarely able to identify the core reasons for program success or failure. This literature review attempts to identify strategies and approaches that are effective or ineffective for preventing or reducing externalizing behavior in children and adolescents. (Contains 1 table, 10 footnotes, and 12 endnotes.)