According to electroencephalographical studies on the pathogenesis of epilepsy a hereditary predisposition plays a greater role than believed before. It is far more frequent than the manifest disease. The phenomenon of cerebral excitability is influenced by different genetically determined, differently age- and sexdependent but amongst each other independent factors. When defining the risk of epilepsy for siblings and descendents in practice, one is forced to proceed from the statistical empirical value with all its uncertainty. During the past few years, the figures have become more exact for special types of epilepsy as absence-epilepsy, myoclonic astatic Petit mal, and Impulsiv-Petit mal. Details of these results and their consequences for genetic advice are discussed.