Examples of prevention and intervention models being used in schools were collected, examined, and classified to develop a comprehensive taxonomy of activities. Principals in a national probability sample of schools were surveyed to identify activities their schools had developed to prevent or reduce gang involvement, delinquency, drug use, or other problem behavior, or to promote a safe and orderly school environment. Individuals knowledgeable about prevention or intervention activities in each school were surveyed to obtain detailed descriptions of specific prevention activities and to describe certain features of their school. Teachers and students were surveyed to obtain information on victimization, safety, gang participation, delinquent behavior, school orderliness, and other aspects of school climate. A sample of 1,279 schools was designed to describe schools in the United States. Overall, 7.6 percent of male and 3.8 percent of female secondary-school students reported they had belonged to a gang that had a name and engaged in fighting, stealing, or selling drugs in the last 12 months. Youths who participated in gangs had much lower educational expectations than did other students and were much more likely to be threatened or victimized in school. Gang members were much more likely than other students to have carried a hidden weapon other than a pocket knife, and they were more involved in violence. A statistical model of the extent to which schools have high rates of student self-reported gang participation implies that concentrated poverty and disorganization in the community, public school auspices, receiving students with behavior problems from various sources, and student perceptions that the school is unsafe influence levels of student gang participation. The association of perceptions that the school is unsafe with the gang-participation rate was especially strong. The most common type of program intended to prevent or reduce gang involvement entailed prevention curriculum, instruction, or training. By far the most common type of gang intervention program involved counseling, social work, and psychological or therapeutic intervention. Gang-involved secondary school students were usually less likely to be involved in or exposed to most kinds of gang prevention or intervention programming. Programs that were developed following a formal needs assessment were implemented in significantly stronger form than those not based on a needs assessment. Implications are drawn for school-based gang programs. 36 tables and 176 references