Dementia is affecting an increasing proportion of the population in the developed world. It is important to reach a correct diagnosis of dementia, because this has implications on the treatment. The electroencephalogram (EEG) is, in general, not a sensitive test for detecting dementia and is not recommended in the standard workup of dementia. In spite of this, however, EEG is useful in patients with deteriorating mental status in whom dementia is suspected mainly to rule out delirium, depression, atypical complex partial seizures, and prion disease. An EEG also provides insight into the physiology of different dementia types. The EEG is most useful when interpreted within a well-defined clinical context, such as knowing the patient's degree of cognitive impairment. It is a noninvasive and inexpensive test, and the threshold should be low for ordering it. This article summarizes EEG findings with aging, different dementia types, and conditions masked as dementia.