The hereditary disorders of muscle excitability are now recognized to be caused by defects in the genes encoding muscle ion channels. This led to a new classification of this disease group. The pathophysiology of these disorders has been elucidated on the molecular level to an extent that exceeds the understanding of the disease mechanisms of most other neuromuscular diseases. The seemingly minor variants of the symptom of myotonia were found to be caused by the remarkable difference that either chloride or sodium channel function is impaired. Even more surprising, the basic defects for hyper- and hypokalemic periodic paralysis, often clinically very difficult to distinguish, turned out to be in the sodium and calcium channels, respectively; these channels are considered to have very different functions in muscle physiology. Three new types of myotonic disease, that is, myotonia, fluctuans, myotonia permanens and proximal myotonic myopathy were discovered. An explanation has been provided as to why myotonia congenita may be transmitted as a dominant or recessive trait.