SUMMARY:: The term epileptic encephalopathy is used to describe diffuse brain dysfunction that is caused, at least in part, by some aspect of epilepsy. Early-infantile epileptic encephalopathy (EIEE), West syndrome, late infantile epileptic encephalopathy, and Lennox-Gastaut are four epilepsy syndromes. These epilepsies are also among the most severe with dire consequences including intractable seizures and severe cognitive dysfunction. These epilepsies share several important characteristics: diverse causes; severe and frequent seizures; diffusely abnormal background activity on electroencephalograms that is often profound; medical intractability; and severe consequences for a normal development. Ohtahara proposed that these epilepsies exist on an electroclinical spectrum and that the clinical and electroencephalogram features are dependent on the maturation of the nervous system. One can now add Late Infantile Epileptogenic Encephalopathy (LIEE) or epilepsy with late-onset of epileptic spasms. Recently, similar gene mutations have been found in several different epilepsy syndromes, reinforcing the notion that these epilepsies are not likely to be distinguished based on cause alone. Recognition and accurate classification of these severe epilepsies is important as the first step toward improving treatment and outcomes.