Aggression is a common neuropsychiatric sequela of traumatic brain injury (TBI), one which interferes with rehabilitation efforts, disrupts social support networks, and compromises optimal recovery. Aggressive behavior raises critical safety concerns, potentially placing patients and care providers in harm's way. Such aggression may be directed outwardly, manifesting as assaultive behavior, or directed inwardly, resulting in suicidal behavior. Given the frequency of TBI and posttraumatic aggression and the potential medicolegal questions surrounding the purported causal relationships between the two, forensic psychiatrists need to understand and recognize posttraumatic aggression. They also must be able to offer cogent formulations about the relative contributions of neurotrauma versus other relevant neuropsychiatric factors versus combinations of both to any specific act of violence. This article reviews the relationships between TBI and aggression and discusses neurobiological and cognitive factors that influence the occurrence and presentation of posttraumatic aggression. Thereafter, a heuristic is offered that may assist forensic psychiatrists attempting to characterize the relationships between TBI and externally or internally directed violent acts.