The primary aim of this study was to explore motivations underpinning aggression among men detained within conditions of high security. Thirty men residing at a high secure psychiatric hospital completed self-report measures, including the Aggression Motivation Questionnaire, Revised EXPAGG and Barratt Impulsiveness Scale-IIr. The Historical items of the Historical, Clinical and Risk-Management (HCR-20) and the Psychopathy Checklist-Screening Version were rated. A subsample of participants agreed to complete a functional assessment on an aggressive incident that had occurred during their placement (n = 9). Increased psychopathy and impulsivity, and the presence of historical risk items were predicted to associate with higher levels of both aggression motivation and beliefs supportive of aggression. Young age at first violent incident and personality disorder related positively to aggression motivation. Thematic analysis conducted on the functional assessments identified social recognition, emotion regulation, communication and protection as functions underpinning aggression. Results are discussed with regards to their implication for violence treatment and assessment, with a focus on motivation recommended.