Leukodystrophies are genetic metabolic diseases which generally occur in early childhood at the time of myelination. Surprisingly, these diseases can also occur during adulthood. Adult forms have various clinical presentations which reflect degenerative diseases of the nervous system. The course may last for decades. This contribution describes the main features of adult leukodystrophies and indicates those for which a biochemical and molecular diagnosis are possible. Other articles deal with their differential diagnosis of leukoencephalopathies and with the diagnostic strategy.