Mindfulness-based interventions (MBIs) in schools have positive effects on cognitive and socio-emotional processes, but do not improve behavior and academic achievement. The use of mindfulness-based interventions (MBIs) in schools has been on the rise. Schools are using MBI's to reduce student stress and anxiety and improve socio-emotional competencies, student behavior and academic achievement. MBIs have small, positive effects on cognitive and socio-emotional processes but these effects were not seen for behavioral or academic outcomes. The studies are mostly of moderate to low quality. Therefore, further evidence from independent evaluators is needed to be able to evaluate the effectiveness of this type of intervention. With the diverse application and findings of positive effects of mindfulness practices with adults, as well as the growing popularity with the public, MBIs are increasingly being used with youth. Over the past several years, MBIs have received growing interest for use in schools to support socioemotional development and improve behavior and academic achievement. This review examines the effects of school-based MBIs on cognitive, behavioral, socio-emotional and academic achievement outcomes with youth in a primary or secondary school setting. MBIs are interventions that use a mindfulness component, broadly defined as "paying attention in a particularly way: on purpose, in the present moment, non-judgmentally," often with other components, such as yoga, cognitive-behavioral strategies, or relaxation skills training. Included studies used a randomized controlled trial, quasi-experimental, single group pre-post test or single subject design and reported at least one of these outcomes: cognition, academic performance, behavior, socio-emotional, and physiological. Study populations include preschool, primary and secondary school students. A total of 61 studies are included in the review, but only the 35 randomized or quasi-experimental studies are used in the meta-analysis. Most of the studies were carried out in North America, and others in Asia, Europe and Canada. All interventions were conducted in a group format. Interventions ranged in duration (4-28 weeks) and number of sessions (6-125 sessions) and frequency of meetings (once every two weeks to five times a week). MBIs have a small, statistically significant positive effect on cognitive and socio-emotional outcomes. But there is not a significant effect on behavioral and academic outcomes. There was little heterogeneity for all outcomes, besides behavioral outcomes, suggesting that the interventions produced similar results across studies on cognitive, socio-emotional and academic outcomes despite the interventions being quite diverse. Findings from this review indicate mixed effects of MBIs in schools. There is some indication that MBIs can improve cognitive and socio-emotional outcomes, but no support for improvement in behavior or academic achievement. Despite the growing support of MBIs for adults, youth may not benefit in the same ways or to the same extent as adults. While not well studied, anecdotal evidence indicates costs and adverse effects of these types of interventions that should be better studied and weighed against the small to no effects on different types of outcomes when considering adoption of MBIs in schools. These findings should be read with caution given the weakness of the evidence produced by the studies. The high risk of bias present in the studies means that further evidence is needed to evaluate the effectiveness of this type of intervention. The evidence from this review urges caution in the widespread adoption of MBIs and encourages rigorous evaluation of the practice should schools choose to implement it. The following are appended: (1) Documentation of search strategies in electronic databases; (2) Data extraction form; (3) Characteristics of included studies: RCT and QED studies; (4) Characteristics of single group pre-post test studies; (5) Characteristics of single subject design studies; (6) Excluded studies; (7) Risk of bias table; (8) Cognitive outcomes by study included in meta-analysis; (9) Academic outcomes by study included in meta-analysis; (10) Behavioral outcomes by study included in meta-analysis; (11) Socioemotional outcomes by study included in meta-analysis; (12) Risk of bias by study; and (13) Funnel plots. [The Campbell Collaboration Education Coordinating Group provided financial support for this report.]