EEG is a non-invasive examination technique that records bioelectrical activity from brain cells through electrodes affixed to the scalp and does not apply any electrical stimulation to the patient. There is no significant discomfort to the patient during the examination. Patients with epilepsy will have an EEG many times during their lifetime, but it is not harmful no matter how many times it is done. Many patients have a concern about having an EEG: can abnormal discharges be captured when they are not having a seizure? The answer is: most patients can. Epilepsy is a chronic disease with recurrent seizures. There are abnormal discharges in the brain tissue during the interictal period, which are usually scattered and do not cause clinical seizures. When these discharges gradually enhance and accumulate to a certain level, they will cause clinical seizures. Because of the short duration of outpatient EEG, most patients with epilepsy obtain an interictal EEG when they have an outpatient EEG. For patients with diagnosed epilepsy treated with medical medication, an intermittent EEG is sufficient. If the patient has clinical seizures but there is insufficient evidence to diagnose epilepsy or if the patient with intractable epilepsy requires precise localization of the epileptic focus for surgical treatment, continuous long-range video EEG testing is required.