Idiopathic epilepsies are considered to be genetically determined. The inheritance can be monogenic and the detected mutation considered sufficient to cause the phenotype. In contrast, when the inheritance is complex, the epileptic phenotype is determined by several minor genetic defects that are much more difficult to discover. In recent years, an increasing number of mutations, mainly associated with rare monogenic idiopathic epilepsy syndromes, have been identified in genes encoding subunits of voltage- or ligand-gated ion channels. A few mutations have also been found in the frequent classical forms of idiopathic generalized epilepsies which are thought to follow a complex genetic trait, for example, in absence or juvenile myoclonic epilepsies. Functional studies characterizing the molecular defects of the mutant channels point to an important role of GABAergic synaptic inhibition in the pathophysiology of idiopathic epilepsies. As a result of genetic and functional investigations, not only will the pathophysiology of epilepsy be better understood, but newly discovered genes and pathophysiological pathways may also determine novel targets for pharmacotherapy, as has been shown for the anticonvulsant drug retigabine, which enhances the activity of neuronal KCNQ potassium channels.