How do school resources affect students’ academic achievement? This chapter provides a survey of economists’ work on the effect of expenditure and class size on student achievement using different international student achievement tests, with a particular focus on the use of quasi-experimental research methods to address challenges of the identification of causal effects. Overall, the international evidence provides little confidence that quantitative measures of expenditure and class size are a major driver of student achievement, across and within countries. The cross-country pattern suggests that class size is a relevant variable only in settings with low teacher quality. Among other school inputs, descriptive evidence suggests that measures of the quality of inputs and, in particular, teachers are more closely related to student outcomes.