Research shows that aggression is an important associated feature of attention-deficit/hyperactivity disorder (ADHD) and is important in understanding the impact of the disorder and its treatment. The occurrence of aggressive behavior in combination with ADHD does not appear to be spurious and the severity and/or presence of aggression and ADHD may significantly impact long-term prognosis. This article defines subtypes of aggression in relation to ADHD, identifies individual differences contributing to aggressive behavior in children with ADHD and discusses selected possible underlying mechanisms of aggression in ADHD, as well as current and emerging treatment approaches. Although aggressive behavior in children with ADHD is common, the reasons for this are not yet well understood. Multidisciplinary research should focus on investigating underlying mechanisms related to aggression in ADHD, as well as the utility of various treatment modalities.