Over the past decade, mindfulness-based interventions (MBIs) have grown in popularity, which has spurred researchers and educators to apply MBIs within the school setting. Although studies have documented positive findings associated with the use of MBIs with youth, more research is needed evaluating current established mindfulness-based curricula that are frequently used in school settings. Therefore, the current study evaluated the effectiveness of the "Mindful Schools" curriculum for increasing the on-task behaviors of students in elementary school classrooms. Stress ratings were also examined, as well as teacher and student acceptability and treatment integrity. Ninety-nine third and fourth-grade students and their classroom teachers served as participants. A multiple-probe, concurrent baseline design across classrooms was used to assess the effectiveness of the intervention on the on-task rates of the students in the classroom. The 15-minute mindfulness lessons were taught twice a week in each classroom over the course of 8 weeks. Systematic direct observation was used to measure on-task behaviors with 30-minute observations occurring immediately after each lesson. Following this observation, students rated their stress levels on a 5-point Likert scale. Visual analysis showed an increase in the level and the trend of on-task behaviors following the implementation of the "Mindful Schools" curriculum. This finding was shown across all four classrooms. Further, data showed a small treatment effect for stress, with three out of the four classrooms showing a decrease in student reported stress ratings. Overall, ratings from teachers and students indicated that the intervention was viewed favorably. Future studies should compare the effects of the "Mindful Schools" curriculum, with other interventions used in the schools. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]