Myotonic disorders are characterised by slowed muscle relaxation and myotonic discharges in the electromyogram. “Pure” myotonic disorders affect only muscle and can be separated into ion channel disorders affecting either the chloride channel (myotonia congenita Thomsen or myotonia congenita Becker) or those affecting the sodium channel (paramyotonia, hyperkalemic periodic paralysis, and myotonia fluctuans). The genetic defects in these disorders are point mutations or deletions within the respective channel genes. A second group of myotonias consists of multisystem disorders with muscle weakness and atrophy plus extramuscular symptoms and signs including cardiac arrhythmias, cataracts, hypogonadism, and pain. Classic myotonic dystrophy (Steinert's disease or DM 1), and proximal myotonic myopathy (PROMM or Ricker's syndrome) are the major syndromes. PROMM is characterised by predominantly proximal muscle weakness and myalgias. Similarly to DM 1, anticipation also occurs in PROMM, but the disease course is usually milder. Steinert's disease belongs to the group of trinucleotide repeat-associated disorders. The DM 1 mutation is an unstable CTG trinucleotide repeat expansion on chromosome 19q13.3 which is diagnostic for the disease. A number of families with PROMM have been linked to a gene locus on chromosome 3q, but the mutation is still unknown. Therefore, direct molecular genetic testing for PROMM is not yet possible.