The cardinal eye manifestations of mtDNA diseases are ophthalmoplegia, optic neuropathy, and pigmentary retinopathy. A number of other eye structures may also be affected in these disorders and the ophthalmologist is in a unique position to detect and interpret these findings. The presence of these ophthalmologic manifestations may be the first clue that the patient has an underlying mitochondrial disease with the eye as the initial or most prominently affected organ. The phenotypic manifestations of mitochondrial disease are protean and variable, and there are no clear-cut, minimal features that define these disorders. The possibility of a mitochondrial disorder should be raised when any of the mitochondrial eye manifestations (Table 1) are present, either alone or in concert with the neurological and systemic (Table 2) manifestations of mitochondrial disease. A maternal family history of an ophthalmologic, neurological, or systemic illness is also compatible with a mitochondrial disorder. The ophthalmologist should not loose sight of the fact that mitochondrial disorders have systemic manifestations and implications, even when the primary manifestations are ocular. Detection of and proactive intervention into comorbid features (eg, diabetes mellitus, hearing loss, heart block) is an important component.