The value of the electroencephalogram in the clinical treatment and understanding of epilepsy and epileptoid conditions has been discussed on the basis of over 400 examinations conducted on patients with various neurological and psychiatric disorders during the past two and one-half years. The E. E. G. was found valuable in the differential diagnosis of epileptic as opposed to other types of clinical "spells" but it was felt that the diagnosis of epilepsy should be made on the basis of a combination of E. E. G. and clinical criteria since epileptiform seizure waves are found in the E. E. G. of patients who are not subject to paroxysmal loss of consciousness or to convulsive movements. The localizing signs and the form of seizure waves in the E. E. G. should be important in questions of surgical intervention. The continuation of less severe epileptiform activity as revealed by the E. E. G. taken when there are no clinical signs of convulsive activity appears to bear some relation to the amount of personality disorder or deterioration in epileptic patients. The discovery of seizure waves in the E. E. G. from patients who had not hitherto been suspected of epilepsy makes possible a definite diagnosis of epileptoid disorders in patients not subject to overt paroxysmal attacks. The E. E. G. provides a sensitive indicator of the effects of anti-convulsive therapy. The relation of these findings to our conceptions of the neuro-physiological mechanism of epilepsy is discussed.