This paper quantifies the extent of student-college "academic undermatch," which occurs when a student's academic credentials permit them access to a college or university that is more selective than the postsecondary alternative they actually choose. Using a nationally representative dataset, we find that 41 percent of students undermatch in their postsecondary choice. We also find that academic undermatch affects students with a range of academic credentials, but is more common among those students from low socioeconomic status families, who live in rural areas, and whose parents have no college degree. Finally, we show that between the 1992 and 2004 high school senior cohorts, academic undermatch has decreased by nearly 20 percent. The decrease is partially due to students being more likely to apply to a matched college. (Contains 8 tables and 3 figures.)