Beyond cases reported to authorities, little knowledge exists on the types, amount, and effects of childhood victimization. Through a national survey of adolescents, researchers examined the prevalence of sexual assault, physical assault, physically abusive punishment, and witnessing an act of violence and subsequent effects on mental health, substance use, and delinquent behavior problems. Gender- and racial/ethnic specific findings are translated into national estimates. Results reveal that rates of interpersonal violence and victimization of 12- to 17-year-olds in the United States are extremely high, and witnessing violence is considerably more common. Black and Native American adolescents were victimized more than whites, Hispanics, and Asians in each type of victimization. Much of the violence experienced by youths is perpetrated by peers or someone the victim knows well. Most sexual assaults (86 percent) and physical assaults (65 percent) went unreported. A clear relationship exists between youth victimization and mental health problems and delinquent behavior. For example--negative outcomes in victims of sexual assault were three to five times the rates observed in nonvictims, and girls who witnessed violence were nearly twice as likely as boys to experience posttraumatic stress disorder. (GCP)