Epilepsy is a disease of the brain characterized by recurring unprovoked epileptic seizures, caused by a transient abnormality of neuronal activity which results in synchronized electrical discharges of neurons within the central nervous system (CNS). This chapter focuses on the most important characteristics of voltage- and ligand-gated ion channels, their role in determining neuronal excitability, and the impact of some reported mutations on epileptogenesis in idiopathic epilepsies. It describes the importance of the thalamocortical loop and thalamic ion channels for the generation of generalized seizures. The binding of transmitters and the coupling to channel opening are complex processes which can consequently be influenced by amino acid changes in many different regions of these channels. Most anticonvulsant drugs that are in clinical use today act by modulating the function of ion channels and the chapter describes how ion channel function can be altered by genetic defects associated with idiopathic epilepsies.