To review recent data on mitochondrial diseases with emphasis on their neuro-ophthalmic manifestations. Numerous studies have associated mitochondrial diseases with neuro-ophthalmic manifestations. Although there has been an explosion of studies on the genetics of mitochondrial diseases over the past few years, pathogenesis is only partly understood and therapy remains very limited. Over the past year, new mutations in Leber's hereditary optic neuropathy have been reported, and at least three genes associated with autosomal dominant chronic progressive external ophthalmoplegia have been described. These findings allow a better definition of the specific genetic mutations and gene products as well as pathophysiology of Leber's hereditary optic neuropathy and chronic progressive external ophthalmoplegia. The current development of animal models allows a better understanding of the pathophysiology of human mitochondrial diseases. The afferent and efferent visual pathways within the central nervous system are frequently involved in mitochondrial diseases. Neuro-ophthalmic signs figure prominently and may be the presenting or even sole manifestation of these disorders. The four most common neuro-ophthalmic abnormalities seen in mitochondrial disorders are bilateral optic neuropathy, ophthalmoplegia with ptosis, pigmentary retinopathy, and retrochiasmal visual loss.