United Nations development goals have consistently placed a high priority on the quality of education—and of learning. This has led to substantive increases in international development assistance to education, and also to broader attention, worldwide, to the importance of children’s learning. Yet, such goals are mainly normative: they tend to be averages across nations, with relatively limited attention to variations within countries. This review provides an analysis of the scientific tensions in understanding learning among poor and marginalized populations: those at the bottom of the pyramid (BOP). While international agencies such as UNESCO and OECD often invoke these populations as the “target” of their investments and assessments, serious debates continue around the empirical science involved in both research and policy. The present analysis concludes that the UN post-2015 development goals must take into account the critical need to focus on learning among the poor in order to adequately address social and economic inequalities.