The pathophysiology of epileptic encephalopathies has long been debated. Recently, some authors proposed the new concept of so-called system epilepsies. This hypothesis postulates that system epilepsies are produced by the enduring propensity to generate seizures in different cerebral areas that, alone, are unable to create a specific electroclinical phenotype. This goes beyond the classical dichotomy between focal and generalized epilepsy. Epileptic encephalopathies, in general, have the ideal profile to be considered as system epilepsies, and West syndrome and Lennox-Gastaut syndrome are two of the best examples. Apart from the conventional neurophysiologic methods for studying brain activities and the pathophysiologic mechanisms underlying epileptic syndromes, other new methods of neuroimaging support this hypothesis.