Most epilepsies are categorized under the umbrella term "idiopathic;" these seizure disorders lack a known cause. New genetic technologies are rapidly identifying specific genes responsible for idiopathic generalized epilepsies (IGEs) and are gradually taking the "I" out of "IGE." Ion channel (both voltage- and receptor-mediated) mutations have been linked to a variety of epilepsies considered idiopathic. Gene errors alter excitability in various ways, depending on the mutation, the regional network, and the stage of brain development. The majority of mutations prolong depolarization, favor repetitive firing, and alter neurotransmitter release or postsynaptic sensitivity at central synapses, but the reason for specific seizure types is unclear. Further analyses of these gene mutations and their effects on the developing brain are providing critical clues in the search to explain the origin of "idiopathic" epilepsy.