- Worldwide, most school peer aggression programs and interventions achieve inconsistent and very modest results which are often temporary at best. In this dissertation, we assumed that many of these peer aggression programs were not evidence-based in their development and were not properly put into practice by teachers. We assumed furthermore, from a behavioral ecological point of view, aggressive behavior not only depends on individual child characteristics but on specific context characteristics as well. School-based peer aggression programs can presumably improve their effectiveness by considering influences, circumstances, and peer relationships from social contexts other than school. One of these non-school contexts is the sports club that, however, has received scant attention with respect to peer aggression and victimization. This is remarkable considering the fact many children belong to sports clubs. We assumed that in sports clubs, just as in schools, children are exposed to peer aggression and perhaps even more than in school. To test these overarching assumptions, we conducted a partially comparative study. Insight into the pervasiveness and constancy of patterns of aggressive behavior in different contexts is key to the development of adequate community-oriented school peer aggression interventions and sports club-specific peer aggression prevention programs. The general aim of this dissertation was to contribute to the knowledge on peer aggression and victimization in different contexts and on the way elementary schools and sports clubs can help prevent and combat aggressive behavior. To this end, we evaluated the potential effectiveness of ten Dutch anti-bullying programs for elementary schools based on an analysis of general theoretical and methodical conditions for effectiveness (Chapter 2). Then we took inventory of the prevalence and stability of peer aggression and victimization for fourth, fifth, and sixth grade students in both Dutch regular elementary schools and Dutch sports clubs. Self-reports from 1,534 elementary students who were either currently participating in a sports club or had recently quit a sports club were gathered with respect to their peer aggression and victimization experiences and to their aggressive behavior roles (i.e., perpetrator, victim, aggressive victim, not involved), at both the sports club and elementary school (Chapter 3).In Chapter 4, with self-reports, the prevalence and stability of peer aggression, prosocial behavior, and resource control strategies for 1,425 children participating in three types of sports (martial arts, contact, and noncontact sports) were examined in the two contexts. We distinguished aggressive children with (i.e., Machiavellians) and without prosocial tendencies (i.e., coercive-aggressive children).Because of their prosocial skills, Machiavellians are difficult for adults to trace and monitor as perpetrators of peer aggression. For the studies in Chapter 3 and 4, we used the 'Dealing With Other Kids Questionnaire' ('Omgaan Met Elkaar Vragenlijst'), our Dutch translation of the SEQ-S from Crick and colleagues. Finally, with in-depth interviews we gathered data on 98 Dutch sports coaches’ views and practices regarding prevention and reduction of these aggressive behaviors in their sports clubs. These views and practices were contrasted with those of a reference group of 96 elementary school teachers and analyzed qualitatively (Chapter 5).