This review explores the alternative functions of mitochondria inside the cell. In a general picture of mitochondrial functioning, the importance and uniqueness of these intrinsic functions make them irreplaceable by other intracellular compartments. Among these are, participation in apoptosis and cellular proliferation, regulation of the cellular redox state and level of second messengers, heme and steroid syntheses, production and transmission of a transmembrane potential, detoxication and heat production. In most of the listed functions, reactive oxygen species modulate a number of non-destructive cellular activities. Some of the mitochondrial functions are reviewed in detail.