While calls to reduce class size in school have considerable popular appeal, the related discussion of the scientific evidence has been limited and highly selective. The evidence about improvements in student achievement that can be attributed to smaller classes turns out to be meager and unconvincing. In the aggregate, pupil-teacher ratios have fallen dramatically for decades, but student performance has not improved. Explanations for these aggregate trends, including more poorly prepared students and the influence of special education, are insufficient to rationalize the overall patterns. International comparisons fail to show any significant improvements for having smaller pupil-teacher ratios. Detailed econometric evidence about the determinants of student performance confirms the general lack of any achievement results from smaller classes. Finally, widely cited experimental evidence actually offers little support for general reductions in class size. In sum, while policies to reduce class size may enjoy popular political appeal, such policies are very expensive and, according to the evidence, quite ineffective. (Contains 29 references.) (Author/DFR)