Mitochondria play a key role in energy metabolism and many other cell physiological and molecular functions. The study of mitochondrial function is crucial for analysis, diagnosis and treatment of numerous pathologies associated with mitochondrial dysfunction, such as oxidative stress, neurodegenerative and inflammatory diseases, metabolic disorders, apoptosis, and in aging. It is, therefore, important to study mitochondrial physiology in states of health and disease in various experimental models and in humans.