Electroencephalography is the only diagnostic instrument directly reflecting cortical neuronal functioning, and it remains an important clinical tool in the diagnosis and differential diagnosis of Alzheimer's disease (AD). Although a normal EEG is found in many patients with mild AD, a pathological EEG is an important finding, because such a result is not in line with differential diagnoses such as depression or pseudodementia within a dissociative disorder. The vast majority of patients with moderate to servere AD have a pathological EEG. A normal EEG in this patient group is more in line with subcortical dementia or frontal lobe degeneration than with AD. Compared to SPECT or routine structural brain imaging (cCT, MRT), EEG has a comparable diagnosis sensitivity and a higher specificity. For monitoring changes of brain function by serial recordings (e.g., during therapy with antidementia drugs), EEG is the best available method.