Ion channels are part of a large family of macromolecules whose functions include the control and maintenance of electrical potential across cell membranes, secretion and signal transduction. Close inspection of the physiological processes involved in channel function and the secondary structure of various ion channels has served as a basis for subdividing ion channels into a number of superfamilies. The voltage-gated ion channels are one of these superfamilies. Recent work has shown that mutations in various ion channel genes are responsible for a number of neuromuscular and neurological disorders. Correlation of the various mutations with the clinical phenotype is providing us with insight into the pathophysiology of these channel proteins. Interestingly, different mutations within the same gene may cause quite distinct clinical disorders, while mutations in different channel genes may result in very similar phenotypes (genetic heterogeneity). Examples of phenotypic variation and genetic heterogeneity are presented in the context of the periodic paralytic disorders of skeletal muscle, episodic ataxia, migraine, long QT syndrome and paroxysmal dyskinesia. Some of these disorders are known to be caused by mutations in ion channel genes, while in the episodic movement disorders, ion channel genes are considered excellent candidate genes.