An EEG is a test that records brain activity. During the procedure, electrodes consisting of small metal discs with thin wires are pasted onto the scalp. The electrodes detect tiny electrical charges that result from the activity of brain cells. The charges are amplified and appear as a graph on a computer screen. Patients are then exposed to a variety of external stimuli, such as bright or flashing lights or noises. The patient is asked to open and close his or her eyes, or to hyperventilate, i.e. breathe deeply for a few minutes. The electrodes transmit the resulting changes in brain wave patterns. Since movement and nervousness can change brain wave patterns, patients usually recline in a chair or on a bed during the test. This test can take one hour or more depending on the doctor's order. No pain is associated with EEG.

An EEG is one of the main diagnostic tests for epilepsy. It may also be helpful in diagnosing other brain disorders such as brain damage related to head injuries, inflammation of the brain, and metabolic and degenerative disorders. It is also used to confirm brain death in patients on life support. An EEG can't measure intelligence or detect mental illness.