Mitochondria are the only intracellular organelles that have their own DNA. The main consequences of damage to their genome are a decrease in the formation of ATP in cells, a violation of intracellular calcium metabolism and an increase in the number of active forms of oxygen in cells. A deficiency in the energy supply of organs and tissues with mitochondrial cytopathies can affect the onset, clinical symptoms and prognosis of diseases. The early clinical manifestations of mitochondrial insufficiency contributing to the occurrence and change of the course of various diseases as a rule do not correspond to the known mitochondrial syndromes and can manifest subclinically. In such cases, the clinical manifestations of mitochondrial cytopathies, which are the basis for diagnostic findings, are masked by symptoms of the underlying disease. Evidence for the presence of mutant DNA in mitochondria should be alarming, but not always confirm the diagnosis. Existing methods for diagnosing mitochondrial disorders with a wide range of therapeutic diseases are not perfect, further studies are needed, including the creation of experimental models that allow us to assess the influence of various factors on the functioning of mitochondria.