This report provides an overview and discussion of the past decade of academic evidence on the causal effects of resources in schooling on students' outcomes. Early evidence lacked good strategies for estimating the effects of schools resources, leading many people to conclude that spending more on schools had no effect. More recent evidence using better research designs finds that resources do matter, but the range of estimates of the impacts is quite wide. The review devotes special attention to differences across the early years, primary and secondary phases. Theoretical work has indicated that interventions early in a child's life may be more productive than interventions later on. However, although there are more examples of good quality studies on primary schooling, the existing body of empirical work does not lead to a conclusive case in favour of early interventions. Three appendices present: (1) Differences in characteristics between high and low-resource schools; (2) Benchmarking and Comparing the Size and Strength of Effects Across Different Studies; and (3) An illustration of the theoretical model (one in which events in one period depend on events in previous periods) used by Nobel-laureate, James Heckman in his argument for the efficacy of early investment in children over investment at a later stage. [This work was commissioned by Ofsted as part of the "Access and Achievement in Education--20 Years On" project.]