In response to the growing concern about equity issues and their implications for overall development, WHO established the Commission on Social Determinants of Health (CSDH) in 2005, which focused on the "social justice" or human rights arguments for health investments. CSDH investigated the factors involved in the so-called "social gradient in health", which refers to the large observable differences in health outcomes within and between countries that are determined by avoidable inequalities in the access to resources and power. CSDH aimed to further investigate the causes of health inequities, with a deliberate detachment from economic considerations, and provide advice on how to tackle them effectively. CSDH also reviewed evidence for action on a wider scope of interventions than CMH, many of which require intersectoral collaboration or advocacy. With CMH and CSDH having adopted different but perhaps complementary standpoints, it soon became clear that greater synergies had to be forged between the two. This WHO resource book on the economics of social determinants of health and health inequalities seeks to begin to build a bridge between the two approaches by explaining, illustrating and discussing the economic arguments that could (and could not) be put forth to support the case for investing in the social determinants of health on average and in the reduction in socially determined health inequalities. The resource book has two main objectives: to provide an overview and introduction into how economists would approach the assessment of the economic motivation to invest in the social determinants of health and socially determined health inequities, including what the major challenges are in this assessment; to illustrate the extent to which an economic argument can be made in favour of investment in three major social determinants of health areas: education, social protection, and urban development and infrastructure.