Progressive external ophthalmoplegia comprises many different disorders. Those of childhood onset can be separated from juvenile or adult onset. Among those of later onset the most common causes are oculopharyngeal muscular dystrophy, oculopharyngodistal muscular dystrophy and the several mitochondrial disorders, especially those with large deletions of mitochondrial DNA (mtDNA) (sporadic), those with maternal inheritance (point mutations), or the autosomal dominant forms with multiple deletions of mtDNA. Ophthalmoplegia of presumably neurogenic origin is seen in some of the familial spinocerebellar ataxias. Advances in molecular genetics should provide information about affected gene products and, therefore, pathogenesis.