Tracking students into different classrooms according to their prior academic performance is controversial among both scholars and policymakers. If teachers find it easier to teach a homogeneous group of students, tracking could enhance school effectiveness and raise test scores of both low- and high-ability students. If students benefit from learning with higher-achieving peers, tracking could disadvantage lower-achieving students, thereby exacerbating inequality. Debates over tracking reached their high point in the United States in the 1990s. An influential report published in 1998 by the Thomas B. Fordham Foundation argued that the available research did not support the contention that tracking doomed impoverished students to inferior schooling, nor did it support universal adoption of the practice. Over the last decade, patterns in grouping students have changed markedly in the U.S.; high school students are no longer placed in rigidly defined general-education or noncollege tracks but have the flexibility to move between course levels for different subjects. These changes may have assuaged some critics, but the broader debate over tracking remains unsettled. The authors shed light on these issues using data from Kenya. In 2005, each of 140 primary schools in western Kenya received funds from the nongovernmental organization International Child Support (ICS) Africa to hire an extra teacher. One hundred twenty-one of these schools had a single 1st-grade class and used the new teacher to split the students into two classes. In 61 randomly selected schools, students were assigned to classes based on prior achievement as measured by test scores. In the remaining 60 schools, students were randomly assigned to one of the two classes, without regard to their prior academic performance. The results showed that all students benefited from tracking, including those who started out with low, average, and high achievement. At the tracking schools, the test scores of students who started out in the middle of their class do not seem to be affected by which section (top or bottom) the students were later assigned to. In other words, any negative effects of being with lower-achieving peers were more than offset in tracked settings by the benefit of the teacher being able to better tailor instruction to students' needs. (Contains 2 figures.)