Disability following head injury varies depending on injury mechanism, neuropathology, and other factors, including medical complications. Mild head injury (Glasgow Coma Scale score 13-15) has been shown to have considerable variability in outcome. Some persons experience rapid symptom resolution whereas others continue to evidence symptoms for an extended duration. A small, but clinically significant number of patients may be neuropsychologically and occupationally disabled at least up to 1 year postinjury. Methodological problems continue to plague mild head injury outcome studies. In contrast, moderate (Glasgow Coma Scale score 9-12) and severe head injury (Glasgow Coma Scale score 3-8) result in more consistent patterns of disability following injury. In general, patients who sustain moderate to severe head injury tend to experience persistent and extensive neuropsychological, psychiatric, and occupational impairment. The impact of rehabilitative interventions is variable and dependent on injury severity, intervention type, and outcome criteria.