School-based interventions are generally ineffective for preventing weight gain in children. In 2007, a group from the University of British Columbia, developed a program called Healthy Buddies, that relied on peer mentors to help young children adopt healthy living behaviours. Pilot studies revealed that the program effectively prevented weight gain in children. This trial had never been tested on a large scale using a more scientifically sound study design. We developed a large school-based randomized trial to overcome that limitation and test the hypothesis that that a school-based peer-led healthy living program would reduce adiposity and increase physical activity among children 6-12yrs old.