Between 1940 and 2000 there was a substantial increase in educational attainment in the United States. What caused this trend? We develop a model of human capital accumulation that features a nondegenerate distribution of educational attainment in the population. We use this framework to assess the quantitative contribution of technological progress and changes in life expectancy in explaining the evolution of educational attainment. The model implies an increase in average years of schooling of 24%, which is the increase observed in the data. We find that technological variables and in particular skill-biased technical change represent the most important factors in accounting for the increase in educational attainment. The strong response of schooling to changes in income is informative about the potential role of educational policy and the impact of other trends affecting lifetime income.