Improvements in empirical research standards for credible identification of the causal impact of education policies on education outcomes have led to a significant increase in the body of evidence available on improving education outcomes in developing countries. This paper aims to synthesize this evidence, interpret their results, and discuss the reasons why some interventions appear to be effective and others do not, with the ultimate goal of drawing implications for both research and policy. Interpreting the evidence for generalizable lessons is challenging because of variation across contexts, duration and quality of studies, and the details of specific interventions studied. Nevertheless, some broad patterns do emerge. Demand-side interventions that increase the immediate returns to (or reduce household costs of) school enrollment, or that increase students’ returns to effort, are broadly effective at increasing time in school and learning outcomes, but vary considerably in cost effectiveness. Many expensive “standard” school inputs are often not very effective at improving outcomes, though some specific inputs (which are often less expensive) are. Interventions that focus on improved pedagogy (especially supplemental instruction to children lagging behind grade level competencies) are particularly effective, and so are interventions that improve school governance and teacher accountability. Our broad policy message is that the evidence points to several promising ways in which the efficiency of education spending in developing countries can be improved by pivoting public expenditure from less cost-effective to more cost-effective ways of achieving the same objectives. We conclude by documenting areas where more research is needed, and offer suggestions on the public goods and standards needed to make it easier for decentralized and uncoordinated research studies to be compared across contexts.