Genetic alterations of various ion channels produce heritable cardiac arrhythmias that predispose affected individuals to sudden death. The investigation of such 'channelopathies' continues to yield remarkable insights into the molecular basis of cardiac excitability. The concept of channelopathies is not restricted to genetic disorders; notably, changes in the expression or post-translational modification of ion channels underlie the fatal arrhythmias associated with heart failure. Recognizing the fundamental defects in channelopathies provides the basis for new strategies of treatment, including tailored pharmacotherapy and gene therapy.