This study evaluated the Gentle Warrior Program, a traditional martial arts–based intervention to reduce aggression in children, as it was implemented in three elementary schools. The sample consisted of 254 children in grades 3, 4, and 5 who participated in the Gentle Warrior Program as part of a larger school violence intervention. Results indicated that boys who participated in more Gentle Warrior sessions reported a lower frequency of aggression and greater frequency of helpful bystanding (i.e., helpful behavior toward victims of bullying) over time, relative to boys with less frequent participation. The effect of participation on aggression was partially mediated by empathy. The effect of participation on helpful bystanding was fully mediated by changes in student empathy. No significant results were found for girls. Results of the study provide preliminary support for the use of martial arts–based interventions to address bullying in schools for boys, by teaching empathy, self-control, and peaceful strategies to resolve conflicts. © 2008 Wiley Periodicals, Inc.