The present study investigates the reactions children show when witnessing victimization. It was carried out with 968 children from 59 kindergartens, in which victimization occurred. Effects of age and gender are found on both behaviour supporting the bully and behaviour supporting the victim, as well as helpless behaviour. Moreover, witnessing behaviour varies as a function of bully/victim role: Bullies display more behaviour supporting the (actual) bully; children usually not involved in bullying show more behaviour supporting the victim; and passive victims exhibit more helplessness. Behaviour in the classroom is also related to witnessing reactions: High levels of aggression in the classroom are associated with more reactions supporting the bully, whereas more reactions supporting the victim are found in groups with higher levels of positive conflict solving. The role