The first goal of the current research project was to get more insight in the determinants of reactive aggression, namely psychopathy, as measured with the Psychopathy Checklist-Revised (PCL-R), and implicit attitudes toward violence. The second goal was was to investigate the possible treatment effects of a multi-modal treatment program for violent forensic psychiatric inpatients, consisting of the extended Aggression Replacement Training (ART) and psychomotor therapy (PMT). The results of this thesis supported the validity of the four-factor structure of the PCL-R in Dutch forensic psychiatric inpatients. Furthermore, measurement invariance could be established between scores on the PCL-R of patients with a psychotic disorder and patients with a personality disorder. This indicates that this instrument can be administered in both types of patients and that scores can be compared in a valid way. The study on implicit attitudes toward violence indicated that implicit attitudes toward violence are to some extent related to aggressive behavior, but that they are mainly have an impact on behavior that is related to the inhibition of aggression. Therefore, altering implicit attitudes toward violence in forensic inpatients may be an important goal during their stay in a forensic hospital. Finally, the pilot-study on the effects of PMT indicated that the addition of PMT did not result in a decrease of aggressive behavior, whereas improvements were predominantly found on behaviors that are related to the inhibition of aggressive behavior, such as the awareness of bodily sensations during anger.