Aggressive behavior in mental disorders may occur in childhood in the context of conduct disorder or in adulthood as a leading feature of personality disorders. Those children, who meet the criteria for conduct disorder already in early life (“early starters”) tend to exhibit high levels of aggression throughout development and continuation of violence in adulthood. They exhibit autonomic underarousal and low autonomic responses which have been shown to be predictive of adult antisocial behavior and which have been suggested to act as biological mediators through which genetic influences operate on antisocial behavior. This predictor is stronger in boys without psychosocial disadvantages compared to those boys with unfavorable social backgrounds and may therefore particularly reflect the biology-antisocial behavior relationship. There are several lines of evidence that aggressive behavior at any age is closely related to an individual's capability to regulate emotions. Emotions of anger or fear trigger reactive, impulsive aggression whereas a failure to experience fear, empathy or guilt facilitates instrumental aggression. Brain structures significantly involved in affect regulation, such as the amygdala and hippocampus, have been found to be smaller in early-onset CD boys compared to healthy controls. In emotional challenge tasks these boys exhibited increased amygdalar hyperresponsiveness to emotional stimuli; this finding might reflect a time-limited mechanism of compensation for smaller amygdala volumina in the maturating brain. Data from neuroimaging and electrophysiology will be forwarded to clarify the neurobiological underpinnings of children with conduct disorders and their risk of being the fledging adult violent offenders.