This paper offers an analysis of urban youth gangs and intervention and prevention strategies for schools, and describes an example of a Cleveland (Ohio) school-wide response. After reviewing the scope of the problem and recent attention to gangs and youth violence, the paper looks at why youth join gangs and lists 12 conditions. Another section explains gang growth and development based on the author's 7 years of experience in working with gangs in schools. An exploration of school impact and responses argues the importance of recognition, acknowledgement, and definition. Policy and administrative responses must develop local and specific definitions of gang activity, prompt institutional action and attention, and leadership and cooperation among school and local authorities. School intervention and prevention strategies require strong organizational commitment from both school boards and school administrators, unmistakable policies forbidding weapons on school property and enforcing school safety, staff and parent training, and establishment of cooperative ongoing relationships with police and social agencies serving students. The Cleveland (Ohio) Public Schools' gang intervention and prevention program and its Youth Gang Unit has seen a 26 percent reduction in gang related incidents during the first 8 weeks of Year 2 of the project. (Contains 10 references.) (JB)