Reducing class size is seen as a way of improving student performance. But larger class sizes help control education budgets. The evidence suggests at best a small effect on reading achievement. There is a negative, but statistically insignificant, effect on mathematics, so it cannot be ruled out that some children may be adversely affected. This 2-page document highlights key components of the Campbell systematic review, "Small Class Sizes for Improving Student Achievement in Primary and Secondary Schools." The review examines the impact of class size on academic achievement. It summarises findings from 148 reports from 41 countries. Ten studies were included in the meta-analysis. Findings indicated there is some evidence to suggest that there is an effect of reducing class size on reading achievement, although the effect is very small. There is no significant effect on mathematics achievement, though the average is negative meaning a possible adverse impact on some students cannot be ruled out. [This summary was prepared by the Campbell Collaboration Education Coordinating Group (ECG). For the full report, "Small Class Sizes for Improving Student Achievement in Primary and Secondary Schools: A Systematic Review. Campbell Systematic Reviews 2018:10," see ED592857.]