Virtually all cells in humans depend on mitochondrial oxidative phosphorylation to generate energy, accounting for the remarkable diversity of clinical disorders associated with mitochondrial DNA mutations. However, certain tissues are particularly susceptible to mitochondrial dysfunction, resulting in recognizable clinical syndromes. Mitochondrial DNA mutations have been linked to seizures, strokes, optic atrophy, neuropathy, myopathy, cardiomyopathy, sensorineural hearing loss, diabetes mellitus, and other clinical features. Mitochondrial DNA mutations also may play an important role in aging, as well as in common age-related neurodegenerative disorders such as Parkinson's disease. Therefore, it is becoming increasingly important for clinicians to recognize the clinical syndromes suggestive of a mitochondrial disorder, and to understand the unique features of mitochondrial genetics that complicate diagnosis and genetic counseling.