Electroencephalography (EEG) constitutes an integral part of the diagnostic process in epilepsy. It is the most important method of investigation in the management of epilepsy and has been widely developed in the last few decades. At present, technological development has provided us with the digital EEG and miniaturization of the equipment with which we are able to register as many channels as necessary in any circumstance and for an indefinite period of time, and together with the development of information technology we are now able to obtain rapid and effective analysis of large amounts of data and a reduction in the number of apparatus. These changes have revolutionized EEG. The EEG which has been used as a diagnostic procedure a posteriori, has increasing application in the direct monitoring of cerebral function. Prolonged EEG monitoring or that performed in the ambulatory, in intensive care units and the video EEG are increasingly accessible and necessary tools for the management of epilepsy.