Using data from the National Center for Education Statistics, the authors examined the effect of tracking on the mathematics achievement of 3,900 tenth graders. They found that tracking affects test scores: Higher-track students (who receive more educational resources, e.g., more-experienced teachers) performed better than would be predicted if they had been assigned to a heterogeneous class. Placement in a lower track was associated with a decrease; placement in an average class with a small increase. Thus, although students in the lower tracks would realize achievement gains in a heterogeneous class, this gain would be at the expense of higher-track students. These results raise a fundamental dilemma for educators and policymakers grappling with ways to improve the nation's schools. Whether to detrack, then, depends on how our society weighs the competing notions of equity and efficiency.