The increasing concentration of wealthy students at highly selective colleges is widely perceived, but few analyses examine the underlying dynamics of higher education stratification over time. To examine these dynamics, the authors build an analysis data set of four cohorts from 1972 to 2004. They find that low-income students have made substantial gains in their academic course achievements since the 1970s. Nonetheless, wealthier students have made even stronger gains in achievement over the same period, in both courses and test scores, ensuring a competitive advantage in the market for selective college admissions. Thus, even if low-income students were "perfectly matched" to institutions consistent with their academic achievements, the stratification order would remain largely unchanged. The authors consider organizational and policy interventions that may reverse these trends. (Contains 5 figures, 5 tables, and 10 notes.)