This paper explores two questions: (1) Do private schools produce greater academic achievement or attainment than public schools? and (2) Does this evidence provide guidance on the potential impact of voucher plans? Based on recent experimental evidence, it finds that Catholic elementary schools have modest effects on the mathematics achievement of poor minority students in grades 2-5 but not in grades 6-8 or among nonblack students. The evidence on elementary reading achievement shows no consistent effects. In secondary schools, the nonexperimental evidence shows no consistent effects on achievement. In contrast, the evidence on attainment is strikingly consistent, indicating that Catholic schools increase the probability of high school completion and college attendance, particularly for minorities in urban areas. However, the latter findings are subject to a caveat: Statistical corrections for selection bias may not fully eliminate bias and may even worsen it. Overall, the evidence is instructive regarding the potential impact of small-scale voucher programs, particularly those encouraging attendance in existing Catholic schools. However, the evidence is notably unhelpful in predicting the effects of large-scale voucher programs, particularly the effects of newly created private schools on outcomes or the effects of competition on public schools. (Contains 59 references.) (Author)