Normative developmental equations provide reliable descriptors of brain electrical activity in people 6 to 90 years old. Healthy persons display only chance deviations beyond predicted ranges. Patients with neurological impairment, subtle cognitive dysfunctions, or psychiatric disorders (including dementia and primary depression) show a high incidence of abnormal values. The magnitude of the deviations increases with clinical severity. Different disorders are characterized by distinctive profiles of abnormal values of brain electrical features. Computerized differential classification of some of these disorders can be achieved with high accuracy. Such classification, providing objective corroboration of brain dysfunctions, may be a useful adjunct to psychiatric diagnosis, which relies primarily on subjective clinical impressions. These methods may provide independent criteria for diagnostic validity, evaluations of treatment efficacy, and more individualized therapy.