Teachers and other school personnel have the responsibility to intervene quickly when a case of bullying comes to their attention. However, previous research has mainly focused on evaluating whole-school prevention programs, while targeted interventions on specific bullying cases have received little attention. In this thesis, I study effectiveness of teachers’ targeted interventions in stopping bullying. In Study I, the focus is on intervention failures. It revealed that interventions failed in one out of four cases. Rather than being associated with differences between schools, intervention failures were mostly explained by characteristics of the bullying cases and the students involved. For instance, frequency and duration of the victimization were positively, and the perpetrators’ own antibullying attitudes and their perception of teachers’ and parents’ antibullying attitudes were negatively associated with the intervention failure. In Study II and Study III, the effectiveness of different intervention approaches was examined. Study II showed that targeted interventions were more effective when the schools used program-recommended approaches (confronting and non-confronting) rather than their own adaptation. Confronting and non-confronting approaches were equally effective. Further, the interventions were more effective when follow-up discussions were organized systematically after each intervention. Study III examined the effectiveness of different approaches using an experimental design and demonstrated that on average, the condemning and empathy-raising messages were equally effective at encouraging youth to stop bullying others, and combining both messages was the most effective. The relative effectiveness of the messages was found to depend on students’ level of cognitive empathy: At low levels of cognitive empathy, the condemning message was the least effective, whereas among those with high cognitive empathy, all messages were equally likely to lead to intention to stop bullying.

To conclude, this thesis increased our understanding of the challenges faced by school personnel when intervening in bullying and effectiveness of different approaches. To make intervening more effective, it is important to consider the characteristics of the bullying cases, to adhere to evidence-based methods, and organize follow-ups systematically after each intervention. The optimal strategy to stop bullying seems to be combining the confronting and non-confronting approaches.