Among the many social and emotional learning (SEL) interventions available, mindfulness-based interventions (MBIs) have become increasingly popular, particularly for preadolescent children who were once thought to not possess the metacognitive abilities or cognitive resources to benefit from such training. Although previous research syntheses indicate that MBIs show promise in promoting positive outcomes across a range of domains, the effectiveness of MBIs for preadolescent children may be masked by the effects of older children who have comprised the majority of samples in past meta-analyses. Hence, to better understand the impact of mindfulness-based training on preadolescent children (ages 6-12 years), the present study reviewed treatment effect estimates across a range of outcomes, including mindfulness, attention, metacognition and cognitive flexibility, emotional and behavioral regulation, academic achievement and school functioning, positive emotion and self-appraisal, negative emotion and subjective distress, externalizing problems, internalizing problems, social competence and prosocial behavior, and physical health. Thirty-two studies (n