Increasing class size is one of the key variables that policy makers can use to control spending on education. But the consensus among many education researchers is that smaller classes are effective in improving student achievement. This view has led to a policy of class size reductions in a number of US states, the UK, and the Netherlands. This policy is disputed by those who argue that the effects of class size reduction are only modest and that there are other more cost-effective strategies for improving educational standards. Despite the important policy and practice implications of the topic, the research literature on the educational effects of class-size differences has not been clear. This review reports findings from relevant studies that measure the effects of class size on academic achievement.