Existing studies from the United States, Latin America and Asia provide scant evidence that private schools dramatically improve academic performance relative to public schools. Using data from Kenya--a poor country with weak public institutions--we find a large effect of private schooling on test scores, equivalent to one full standard deviation. This finding is robust to endogenous sorting of more able pupils into private schools. The magnitude of the effect dwarfs the impact of any rigorously tested intervention to raise performance within public schools. Furthermore, nearly two-thirds of private schools operate at lower cost than the median government school. Data on test scores, household survey and school administrative are appended. (Contains 1 figure, 2 tables, and 3 endnotes.)