In this article, the authors investigate group work in a heterogeneous urban high school mathematics classroom. Two questions are explored: How do students describe cooperative group work in their mathematics class? How do students describe the way their socially constructed identities influence the nature of their group interactions in mathematics classrooms? The authors present a case study of the ways in which race, gender, and other social identities might influence the nature of group work in reform-oriented high school mathematics classrooms. The analysis, based on 14 interviews with high school students, focused on students perceptions of group work and their theories about when cooperative groups work well and when they do not. Students named interactional style, mathematical understanding, and friendships and relationships as the most influential factors. Using an analytic lens informed, in part, by critical race theory, the authors highlight the racialized and gendered nature of these factors.