This chapter describes how and why schools categorize and group students into separate classrooms and programs for instruction. In doing so, it addresses one of the core ways that the school's structure influences students' opportunities to learn. The chapter focuses on ability grouping and curriculum tracking, as well as on the multitude of special classes and programs created to respond to differences among students. It explains the socially constructed nature of the categories and labels schools assign to students—labels like special education, gifted, compensatory, and Limited English Proficient—and provides some of the history behind labeling and sorting in American schools. The chapter reviews some of the evidence showing that these practices often do as much to create differences as they do to meet students' different needs. It describes the work of educators who seek to avoid these damaging practices and instead use more multidimensional, developmental, and socially just approaches for meeting students' needs.