Using 18 years of data from more than 1,300 four-year colleges and universities in the United States, we investigate the extent to which institutional characteristics and contextual factors influence the propensity of colleges to indicate that they engage in affirmative action in their admissions decisions. Consideration of race/ethnicity in admissions declined sharply after the mid-1990s, especially at public institutions. Rather than being shaped by specific historical and political contexts, affirmative action in admissions appears to be a widely institutionalized practice in higher education that has been tempered by changes in the policy environment over time. (Contains 5 tables, 3 figures, and 23 notes.)