The aging population of the United States and many other countries guarantees that disorders common in elderly persons will continue to become more prevalent. In the past decade or so, we already have witnessed the beginning of this trend, with the rapid rise in the number of patients with degenerative dementia, especially Alzheimer's disease. On another track, sophisticated and timely medical and surgical interventions have produced a growing number of patients who survive serious head injury. These patients are typically young, and may live for many decades after their brain injury. In both examples, a core feature of the condition is impaired cognition and behavior; that is, defects in higher-level capacities such as memory, intellectual function, language, and decision-making. Neuropsychological assessment cannot be viewed as a luxury in cases such as these but rather, as a vital component of the neurodiagnostic evaluation. The neuropsychologist uses a set of standardized instruments to measure cognition and behavior in a comprehensive and precise manner, yielding rich information that may assist in accurate diagnosis, prudent management, and optimal rehabilitation. The neuropsychological examination may provide the only means with which to detect alterations in brain function, as in cases of mild head injury or low-level toxic exposure, which may not be revealed through other procedures. Finally, neuropsychological assessment will continue to play a key role in the scientific programs that are dedicated to advancing our understanding of the neural substrates of complex behavior.