periodic electroencephalographic (eeg) patterns consist of discharges usually epileptiform in appearance, which occur at regular intervals, in critical patients. they are commonly classified as periodic lateralized epileptiform discharges (pleds), bilateral independent pleds or bipleds, generalized epileptiform discharges (gpeds) and triphasic waves. stimulus-induced rhythmic, periodic or ictal discharges (sirpids) are peculiar eeg patterns, which may be present as periodic discharges. the aim of this study is to make a review of the periodic eeg patterns, emphasizing the importance of their recognition and clinical significance. the clinical significance of the periodic eeg patterns is uncertain, it is related to a variety of etiologies, and many authors suggest that these patterns are unequivocally epileptogenic in some cases. their recognition and classification are important to establish an accurate correlation between clinical, neurological, laboratorial and neuroimaging data with the eeg results.