Clinical impulsivity has been characterized in both dimensional and categorical terms. Whereas DSM-III-R classifies personality disorders characterized by impulsivity and impulse control disorders as discrete entities, impulsive symptoms and traits can also be conceived in terms of an underlying behavioral dimension. The authors review research on impulsivity and the impulse control disorders from a biological perspective. In particular, they critically review evidence that the serotonin neurotransmitter system mediates symptoms and traits of impulsive personality disorders and the impulse control disorders.