In this article, I describe a program of research on the psychosocial benefits of racial/ethnic diversity in urban middle schools. It is hypothesized that greater diversity can benefit students' mental health, intergroup attitudes, and school adaptation via three mediating mechanisms: (a) the formation and maintenance of cross-ethnic friendships, (b) the development of complex social identities, and (c) decreases in perceived vulnerability. These hypotheses are examined in a 3-year longitudinal study of approximately 6,000 ethnically diverse 6th-grade students as they enter one of 26 urban middle schools in California. The overarching goal of this article is to present a strong argument that greater school ethnic diversity can buffer many of the normative challenges of early adolescence.