Leber hereditary optic neuropathy (LHON) is a maternally inherited disease characterized by acute or subacute painless central visual loss usually in young adults, predominantly in males. Except for optic atrophy, LHON patients are usually otherwise healthy. Occasionally, LHON is associated with neurological, cardiac, and skeletal changes. The clinical course of LHON has several stages. Peripapillary microangiopathy is present from the beginning. Microangiopathy disappears as the disease progresses towards the end stages. Simultaneously, the retinal nerve fiber layer fades from view, first papillomacular nerve fiber bundles, and months later, the whole nerve fiber layer becomes atrophic. At the end stage the centrocecal scotoma is large and absolute. Loss of vision is usually permanent, but spontaneous recovery can occur. Despite a few attempts, no effective treatment to prevent or halt LHON has been found. Several mitochondrial DNA (mtDNA) mutations are associated with LHON, but the pathogenic processes leading to optic nerve atrophy are largely unknown. About 15% of the families are heteroplasmic, i.e., both mutant and wild type mtDNA coexist within an individual. The level of heteroplasmy between different tissues can vary markedly. mtDNA mutations are not sufficient to cause visual loss in LHON, since not all individuals harboring a pathogenic LHON mutation express the disease. There are additional genetic and/or environmental precipitating factors, but thus far they are unknown.