Numerous definitions and systems of describing aggression have been reported in the literature. While in some circumstances aggression can be adaptive and necessary for survival, aggressive behaviors can also be out of context or maladaptive. Specifically, maladaptive aggression was defined as being much more intense, frequent or prolonged than would be adequate or adaptive to the individual environmental circumstance. In the past, a system that divides maladaptive aggression into “impulsive, reactive” or “proactive” forms has been favored and has received much support for its internal and external validity. Proactive aggression is premeditated and predatory whereas impulsive aggression (IA) is more reactive, affective, and defensive in nature. IA behavior has been clinically described as “unplanned and overt. The aggressor perceives the outcome of the aggressive act to be negative and with negative accompanying emotions, such as frustration, regret, guilt and fear”. IA may be obvious as early as 4.5 years of age, and childhood physical abuse may be associated with its development. Proactive aggression and IA may co-occur but different neurobiologic pathways have been proposed for each subtype, and treatment of one aggression type does not automatically improve the other. In addition, aggressive behaviors are expressed differently across the genders. These findings suggest distinct etiologic brain mechanisms for IA and other forms of maladaptive aggression.