The KiVa antibullying program has been widely implemented in Finnish comprehensive schools since 2009. The program is predicated on the idea that a positive change in the behaviors of classmates can reduce the rewards gained by the perpetrators of bullying and consequently their motivation to bully in the first place. KiVa involves both universal and bullying specific actions to prevent the emergence of new cases of bullying, stop ongoing bullying, and reduce the negative consequences of victimization. The program has been evaluated in a randomized controlled trial involving 234 Finnish schools and during broad dissemination across Finnish schools (the evaluation involving almost one thousand schools) with positive findings. The program content and the implementation model are presented in this article, and the findings from the evaluation studies are summarized.