The myotonias and periodic paralyses are a diverse group of skeletal muscle disorders that share a common pathophysiological mechanism: all are caused by derangements in the electrical excitability of the sarcolemma. Mutations within coding regions of ion-channel genes have been identified recently as the underlying molecular defects in these heritable disorders. Chloride-channel mutations cause a reduction in the resting conductance, which enhances excitability and gives rise to myotonia. By contrast, missense mutations in the L-type Ca2+ channel reduce the electrical excitability of the fiber and cause a form of periodic paralysis. Mutations of the sodium channel impair inactivation of the channel, which, depending on the type and severity of the functional defect, results in either paralysis or myotonia.