Examined the criterion validity and the utility of Dodge and Coie's (1987) measure of reactive and proactive aggression. Participants were 405 children in kindergarten through 5th grade attending an urban elementary school. Examined criterion validity by testing whether reactive aggression or proactive aggression was significantly correlated with criterion measures of overall impairment as measured by the Impairment Rating Scale (Pelham, Gnagy, et al., 1996), classroom behavior as measured by a frequency count of classroom rule violations, and peer adjustment as measured by teacher ratings of peer behavior on the Pittsburgh Modified IOWA Conners (Pelham, Milich, Murphy, & Murphy, 1989). Examined utility by testing whether reactive aggression or proactive aggression was significantly correlated with criterion measures after controlling for each other and after controlling for attention deficit hyperactivity disorder, oppositional defiant disorder, and conduct disorder as measured by the Disruptive Behavior Disorder Rating Scales (Pelham, Gnagy, Greenslade, & Milich, 1992). Results showed good evidence of criterion validity for both reactive and proactive aggression, but mixed evidence for their utility.