In this article, we review periodic EEG patterns, which have been classified into four different types based on their interval duration (short or long) and topographic distribution (lateralized, bilaterally independent, or diffuse and synchronous). The four patterns are: (1) periodic lateralized epileptiform discharges; (2) bilateral independent periodic lateralized epileptiform discharges; (3) periodic short-interval diffuse discharges; and (4) periodic long-interval diffuse discharges. We also discuss morphology, etiologies, and clinical correlates of each pattern and possible pathophysiological mechanisms of periodicity.