Mitochondrial diseases are extremely heterogeneous multisystem disorders predominantly affecting tissues or organs with high oxygen consumption like skeletal muscles, brain, endocrine glands, myocardium, eyes, ears, intestines, liver, kidneys, and bone marrow. Although various clinical syndromes have been described, they frequently overlap and there is no diagnostic gold standard to identify all. It is difficult to chart the future of an affected individual as also to predict the response to treatment which is mostly supportive and symptomatic. The rapidly increasing understanding of the pathophysiologic background of mitochondrial disorders may facilitate diagnostic approach and open perspectives to curative therapies. With the coming of age for mitochondrial medicine, it is now appropriate that physicians keep themselves well-acquainted with the recent developments in this expanding field of biomedical research.