The results of many studies using quantitative EEG techniques in clinical settings have been published. Those reports are reviewed here, with emphasis on those that used EEG frequency analysis and topographic mapping. In cerebrovascular disease, these methods can confirm the existence of lesions that are too mild to show up on routine EEG or too mild or too early to show up on computed tomography. The results correlate well with cerebral blood flow studies. These EEG tests can be done continuously in an intensive care unit or operating room. However, exact localization ability is inferior to that seen using traditional neuroimaging tests. In epilepsy, quantitative EEG techniques have found subtle degrees of background EEG changes near epileptic foci. Other methods can quantify epileptic spikes in useful ways and can indicate which region is driving other regions during seizures. Quantification is also useful for measuring drug effects when drugs (such as thiopental) are given deliberately to provoke acute EEG changes. Other measurements of drug effects may become useful in the future. In patients with mass lesions and metabolic encephalopathies, quantitative EEG changes do occur, and some of these correlate with the clinical state. However, in the latter settings, the clinical advantages for patient care are not yet clear, especially in comparison to available neuroimaging studies and other routine medical tests. For dementia, quantitative EEG techniques are being developed. Some of these tests are accurate in moderately or severely demented patients, but there is still poor accuracy for early or borderline cases. For dysiexia, schizophrenia, and depression, there is a considerable volume of research reports but still no consensus about how to use quantitative EEG tests for care of individual patients. These tests require substantial user expertise in EEG. At present, these tests should be viewed as adjunctive to traditional EEG testing: such routine EEG testing should serve as the foundation for any clinical use of quantitative EEG tools.