This report systematically reviews research on the outcomes of programs that teach young children in a group setting before they begin kindergarten. Study inclusion criteria included the use of randomized or matched control groups, evidence of initial equality, and study duration of at least 12 weeks. Studies included valid measures of language, literacy, phonological awareness, mathematical, and/or cognitive outcomes that were independent of the experimental treatments. A total of 38 studies evaluating 27 different programs met these criteria for outcomes assessed at the end of preschool and/or kindergarten. The review concludes that on academic outcomes at the end of preschool and/or kindergarten, six early childhood programs showed strong evidence of effectiveness and five had moderate evidence of effectiveness. A few longitudinal studies have followed their subjects into secondary school, and even adulthood. These studies show that comprehensive programs focused broadly on cognitive development rather than solely academic skills had better long-term effects on social adjustment outcomes such as reductions in delinquency, welfare dependency, and teenage pregnancy, and increases in educational and employment levels. (Contains 3 tables.) [Funding for this paper was provided by the CfBT Education Trust.]