This article reviews the current research literature on the development of aggression and callous-unemotional traits. Research suggests there are two functions to aggression, reactive and instrumental, and each has concomitant cognitive and emotional factors associated. Furthermore, callous-unemotional (CU) traits (i.e., an absence of empathy and guilt) have been shown to be associated with the instrumental type of aggression. Research on CU traits suggests that there are distinct developmental mechanisms operating in the development of aggressive and violent behavior for youths with and without these traits. These distinct developmental mechanisms have important implications for the assessment and treatment of aggressive and violent youths.