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. Chapter 6, titled, The social and external benefits of education, reminds that there can be significant differences between the private returns to education and the social returns. Often forgotten in discussions of this distinction is the issue of externalities associated with education. The author presents a comprehensive assessment of such externalities and finds that the impact of education on society is pervasive. The chapter defines the theoretical basis for the identification and estimation of education externalities, presents a taxonomy of education’s public good externalities and reviews estimates in the literature of the overall size and value of these externalities. It also considers estimates of the non-market impacts of education on specific development goals, offers approximations of total social rates of return that these estimates of education externalities imply and identifies the separate private non-monetary returns in order to distinguish them from the public good development outcomes.