Epileptic seizures occur because of the increased excitability of neuronal cells in the brain that is of a mainly genetic origin in at least one third of all epilepsies. The so far identified mutations in inherited monogenic idiopathic epilepsies mainly affect ion channel genes and could be linked to both focal and generalized forms of inherited epilepsy. In a healthy brain, ion channels in the membranes of excitatory and inhibitory neurons are responsible for a neuronal balance. Disruption of this balance by changing the ion channel function can therefore lead to epileptic seizures. In this overview, we describe and discuss known epilepsy-associated mutations in ion channel genes, their functional consequences, and the resulting disease mechanisms.