This study investigated the relationship between preschool children's social-cognitive abilities (theory of mind and social information processing; SIP) and their observed physical and relational aggressive behaviour. Children with more advanced social-cognitive abilities engaged in fewer acts of physical aggression; however, much of the ability of the social-cognitive variables to predict physical aggression was shared with language ability. In addition, social-cognitive understanding moderated the connection between language ability and physical aggression. Exploratory examination of gender differences in patterns of association between physical aggression and the social cognitive understanding variables revealed that the relationships were only true for boys. Relational aggression was not associated with social cognitive ability for either boys or girls, but this is likely due to the low frequency of relationally aggressive behaviour observed in this sample.