This research explored links between cognition (both social and academic) and children's behaviour in a bullying situation (participant roles). Participants were 186 fourth to sixth grade boys and girls from four primary schools in central Greece. Six categories of social cognition (self-efficacy for assertion, self-efficacy for aggression, expectations that aggression will lead to rewards, expectations that aggression will lead to victim suffering, the value placed on rewards and the value placed on suffering in the victim) and two categories of academic cognition (self-efficacy for learning and performance and the self-regulatory strategies used while solving problems) were investigated in connection to six participant roles (bully, victim, assistant, reinforcer, defender and outsider). Results suggest that there are differential associations between cognitions and the roles that children take in bullying situations, according to gender. Academic self-efficacy combined with certain social cognitions predicted both victimisation and bullying behaviour. Self-regulatory strategies combined with social cognitions similar to victim's and bully's predicted both assistant and reinforcer behaviour, while none of the cognition measures predicted defender or outsider behaviour. Implications for interventions against bullying are briefly discussed.