Mutations in cardiac Na(+) and K(+) channels can disrupt the precise balance of ionic currents that underlies normal cardiac excitation and relaxation. Disruption of this equilibrium can result in arrhythmogenic phenotypes leading to syncope, seizures, and sudden cardiac death. Congenital defects result in an unpredictable expression of phenotypes with variable penetrance, even within single families. Additionally, phenotypically opposite and overlapping cardiac arrhythmogenic syndromes can stem from one mutation. A number of these defects have been characterized experimentally with the aim of understanding mechanisms of mutation-induced arrhythmia. Improving understanding of abnormalities may provide a basis for the development of therapeutic approaches.