Over the past decade and a half, the number and proportion of college students opting not to reveal their race when asked have shot up, to 5.9 percent of all students in 2001 from 3.2 percent a decade earlier. The increases have raised two major questions: Who are these students, and why are they declining to identify themselves? The answers have implications for college officials and policy makers on a wide range of issues, including affirmative action and student life.