While male high school graduates were more likely to enroll in college than their female counterparts three decades ago, this pattern reversed by the late 1980s. The gender gap in college attendance rates has continued to increase ever since. In this paper, I examine several factors that might affect the diverging college entrance patterns for young men and women. Using three nationally representative longitudinal data sets of high school students, I find that women's high school performance--as measured by test scores and the number of math and science courses taken--increased more rapidly than men's over the past three decades. A simple decomposition exercise indicates that women's greater advances in high school achievement can account for more than half of the change in the college enrollment gender gap over the past three decades. Using a simple theoretical model, I show that the contribution of women's superior performance in high school to women's increased college attendance combines both the effects of exogenous changes in how high schools prepare women for college "and" changes in high school performance induced by women's optimizing responses to increased labor market opportunities.