Myotonia congenita (Thomsen's disease) is a muscular disorder with autosomal dominant inheritance. The main symptom is muscle stiffness caused by hyperexcitability of the muscle cell membranes. The disorder is noticeable at birth or in early childhood and is progressive. Muscle hypertrophy is a common sign. Recessive generalized myotonia (Becker) resembles myotonia congenita in many respects, but there are also differences, e.g. later onset and transient muscle weakness experienced by many patients during muscle exertion after rest. The clinical signs usually progress over several years and then remain stable. The decisive criterion for the differential diagnosis of Thomsen or Becker type myotonia is the mode of inheritance. Both disorders are caused by defects of the gene, situated on chromosome 7, that is responsible for the normal functioning of the chloride channels in the muscle cell membranes. The differences between the two types of myotonia are probably caused by different mutations of this gene. The present paper also describes two animal models of myotonia (the myotonic goat and the myotonic mouse), which simulate the dominant and recessive types of human myotonia, respectively. The models have been very helpful in research into the origins of myotonia.