The Texas 10% law states that students who graduated among the top 10% of their high school class are guaranteed admission to public universities in Texas. We estimate the causal effects of this admissions guarantee on a sequence of connected decisions: students' application behavior, admission decisions by the university, students' enrollment choices conditional on admission; as well as the resulting college achievement. We identify these effects by comparing students just above and just below the top 10% rank cut off. We assume that other student characteristics and incentives are continuous at this cut off. We find that students react to incentives created by the admissions guarantee—for example, by reducing applications to competing private universities. The effects of the admissions guarantee depend on the university and the type of students it attracts. The 10% law is binding and alters the decisions of the admissions committees. We find little evidence that the law increases diversity or leads to meaningful mismatch for the marginal student admitted. (JEL I23, I28)