A disorder of impulsive aggression has been included in DSM since the first edition. In DSM-III, this disorder was codified as intermittent explosive disorder, and it was thought to be rare. However, the diagnostic criteria for the disorder were poorly operationalized, and empirical research was limited until research criteria were developed a decade ago. Subsequently, renewed interest in disorders of impulsive aggression led to a recent series of community-based studies that have documented intermittent explosive disorder to be as common as many other psychiatric disorders. Other recent research indicates that compared with DSM-IV criteria for intermittent explosive disorder, research criteria for the disorder better identify individuals with elevated levels of aggression, impulsivity, familial risk of aggression, and abnormalities in neurobiological markers of aggression. In addition, other data strongly suggest important delimitation from other disorders previously thought to obscure the diagnostic uniqueness of intermittent explosive disorder. Overall, these data suggest that the diagnostic validity for the integrated research criteria is substantial and is now sufficient for recognition and inclusion in DSM-5.