This article presents a critical review of quantitative electroencephalographic (qEEG) research and issues relevant to its clinical application. Included is a summary of methodologic issues necessary for a reliable implementation of qEEG within clinical settings and a brief discussion of controversial issues surrounding this implementation. The main body of the article includes a summary of qEEG findings that involves adults with various neurologic and psychiatric disorders, which is followed by an in-depth presentation of qEEG research that involves children and adolescents with neurologic and psychiatric disorders. A concluding section presents a qEEG-based neurophysiologic model of attention deficit disorder.