Chapter 1 discusses the different types of aggressive behavior (impulsive, psychotic, and psychopathic), each likely with unique neurobiological substrates. Various risk factors for violence and aggression, especially in mental illness, are also described. These risk factors include childhood maltreatment, substance abuse, cognitive dysfunction, and treatment nonadherence. In conjunction with several available violence risk assessment tools, clinical evaluation of known risk factors should aid clinicians in determining which individuals are most likely to become violent or aggressive.