The literature concerning the effects of electroconvulsive therapy (ECT) upon the EEG is reviewed with respect to the degree and persistence of abnormalities. The most common electrophysiological dysfunction consists of generalized regular and irregular slow wave activity. This slowing typically disappears by a few weeks to a few months following completion of the ECT course but in rare cases may persist for longer periods. Patients given large numbers of ECT treatments tend to show more prolonged alterations. Possible correlations of these EEG changes with a variety of parameters are discussed.