During middle childhood and adolescence, victimisation appears to be a group process involving different participant roles. However, peer reports with younger children (four to six years old) have failed to identify the participant roles of assistant (to the bully) reinforcers or defenders with much reliability. This may be because peer victimisation is a more dyadic process among younger children (behavioural reality), or because of limitations in young children’s cognitive capacity to identify these behaviours (cognitive limitations). The findings of an observational study which examined the group nature of peer victimisation among young children are presented. Observations were made of 56 children aged four and five years using time sampling during free play at school (totalling 43.5 h of observation). Records were made of their behaviour when an onlooker witnessed aggression by others, and also of others’ behaviour when they were being aggressive or being victimised. Although children other than the aggressor and target were present in nearly two thirds of the episodes of peer victimisation observed, few exhibited behavioural responses in line with the assistant, reinforcer or defender roles. This supports the behavioural reality rather than the cognitive limitations explanation. Sex differences were observed in types of aggression displayed by children, with boys more likely than girls to be physically aggressive. Children were less likely to be aggressive to other-sex peers and were most likely to be victimised by children of the same sex as them. There were also sex differences in children’s onlooker behaviour. The implications for our understanding of the development of peer victimisation and bullying in children are discussed.