Mitochondria play essential role in the cell functioning. Besides their obvious importance in overall cell energy production, mitochondria support cellular redox homeostasis, produce small signaling molecules, generate steroids, heme and Fe-S cluster proteins, participate in apoptosis, cell proliferation, detoxication and thermogenesis. Production of reactive oxygen species (ROS) may be regarded as an important mitochondrial function for regulated signaling and destructive processes. Mitochondria themselves seems to be a source, sensor and a target for ROS. Signaling function of mitochondrial ROS is demonstrated on the example of cardioprotective mechanism developed under ischemic preconditioning directed against ischemia/reperfusion damage.