About two million Americans incur head injuries each year, resulting in approximately 50,000 chronic disabilities. Several methods are currently used to determine the physiological changes underlying behavioral sequelae commonly associated with closed head injuries. Structural assessment techniques, including lesion studies, standard radiographs, computed tomography and magnetic resonance imaging, are used to determine the physical integrity of brain structures. Functional assessment techniques, including neuropsychological methods, the electroencephalograph, evoked potentials and positron emission tomography, are used to examine the brain through an analysis of its behavioral, electrical and chemical outputs. Accurate head injury assessment is vital for diagnosis, treatment planning and research. Injuries are commonly divided into the categories of mild, moderate and severe based on duration and depth of coma and the length of posttraumatic amnesia. Application of these severity assessment parameters is currently being improved through the use of standardized assessment measures.