In everyday medicine, the EEG is mostly used to make or verify certain diagnoses. By far the most important job of the EEG is to diagnose the exact type of seizure disorder in epileptics. The EEG also can give hints when it comes to brain tumors or certain abnormal variations of blood vessels within the brain. Especially it is possible to localize a problem: An epileptic, for example, can also have a brain tumor. The EEG can be used to differentiate between both conditions. It can also be used to find out if unusual anatomical structures within the brain are responsible for a seizure, or if it is rather a generalized phenomenon. Certain metabolic diseases also can affect the brain and thus the EEG.
The EEG, finally, plays a role in diagnosing the so-called brain-death. A brain-dead person should not have electrical EEG-activity for 30 minutes in a row, but this is only one criterion.
In medical research, the EEG has been used to analyse sleep and consciousness, especially the brain's reaction on external events like sound- or light-effects.