The current study examined the distinction between reactive and proactive aggression in a sample of detained girls (N = 58) aged 12 to 18. This study employed a self-report measure of aggression that was designed explicitly to assess both the forms that aggression takes (i.e., relational and overt), as well as the functions that aggression serves (i.e., reactive and proactive). Reactive aggression was uniquely associated with poorly regulated emotion and anger to perceived provocation, whereas proactive aggression was uniquely associated with callous–unemotional (CU) traits and biased outcome expectations for aggression. While overt aggression appeared to largely account for these associations, relational aggression showed strong and unique associations with CU traits. The current findings highlight the importance of assessing reactive and proactive aggression, as well as both overt and relational aggression, in detained girls.