The current paper reviews research suggesting that the presence of a callous and unemotional interpersonal style designates an important subgroup of antisocial and aggressive youth. Specifically, callous-unemotional (CU) traits (e.g., lack of guilt, absence of empathy, callous use of others) seem to be relatively stable across childhood and adolescence and they designate a group of youth with a particularly severe, aggressive, and stable pattern of antisocial behavior. Further, antisocial youth with CU traits show a number of distinct emotional, cognitive, and personality characteristics compared to other antisocial youth. These characteristics of youth with CU traits have important implications for causal models of antisocial and aggressive behavior, for methods used to study antisocial youth, and for assessing and treating antisocial and aggressive behavior in children and adolescents.