In order to understand the concepts of seizures, epilepsy and epileptogenesis, we will first consider some of the basic anatomic and electrophysiologic properties of the cerebral cortex, and the factors that determine the level of neural activity at the cellular and cell network level. We will then discuss the physiologic basis of the electroencephalogram (EEG), routinely used in assessing patients with seizures and other neurological disorders. Finally, we will address some of the main features of the abnormal physiological activity that occurs within a seizure focus, and present a few of the proposed mechanisms that may underlie certain seizure types.