The myotonic disorders result from dysfunction in either the chloride or sodium channel and these disorders fall in the category of nondystrophic myotonias. The other group is represented by myotonic dystrophies. The myotonic dystrophies are multisystem, autosomal dominantly inherited, highly variable muscle disease more frequent in adults. So far two distinct entities have been described: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2/PROMM). The latter is similar but distinct from classic, myotonic dystrophy of Steinert. In this review, we will focus on clinical features, genetics, pathophysiology, clinical laboratory tests, and treatment of DM2. Related more frequent myotonic disorders (ie, autosomal-dominant and autosomal-recessive myotonia congenita) will also be described. Sodium channel myotonia and myotonic-like disorders (ie, Schwartz-Jampel syndrome) will not be covered in this review.