The study of the economics of education has expanded considerably in scope over the last four and a half decades – as is evidenced by the wide range of material in this volume. But the key insights of human capital theory remain central to any analysis of the demand for education, and it is therefore appropriate that this is where the volume should begin. Psacharopoulos and Patrinos outline the basics of the human capital model, emphasizing the role that education has to play in raising individuals’ productivity. They also provide empirical estimates of the rate of return to education in a variety of countries. Estimates such as these have been particularly influential in shaping the educational policies of international organizations such as the World Bank, informing in particular the policy of loans to support education in developing countries and the goal of universal primary education.