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The serious consequences of school bullying on children’s physical and mental health call for the implementation of effective intervention programs. The goal of this paper is to present the theoretical background, components, and effects of the anti-bullying program KiVa, which was designed in Finland in 2006 at the request and with funding from the Ministry of Education. It is currently used in 90% of Finnish comprehensive schools and in many countries across the world. KiVa is based on the view that students witnessing bullying play a key role in maintaining or stopping it. Therefore, the program targets all students. In Finland, its effectiveness was first demonstrated by a randomized controlled trial (RCT) and then during its nationwide rollout. KiVa significantly reduces bullying and reinforcement of the bullies’ behavior, and increases empathy for the victims. Furthermore, the program has been found to decrease children’s anxiety and depression, and to improve their school motivation, school achievement, and perception of peers. This paper pays special attention to the factors that mediate and moderate the effectiveness of the program.