This paper offers a systematic critical review of controlled outcome studies of the "Philosophy for Children" (P4C) method in primary (elementary) and secondary (high) schools. Ten studies met the stringent criteria for inclusion, measuring outcomes by norm-referenced tests of reading, reasoning, cognitive ability, and other curriculum-related abilities, by measures of self-esteem and child behaviour, and by child and teacher questionnaires. All studies showed some positive outcomes. The mean effect size was 0.43 with low variance, indicating a consistent moderate positive effect for P4C on a wide range of outcome measures. The implications for practice, policy, and future research were explored, particularly in relation to cost-effectiveness.