The paper deals with the principles of cathodic protection for atmospherically exposed concrete structures, the various protecting effects induced by the cathodic polarization and tests and field experience results. The differences between the cathodic protection applied for controlling the corrosion rate of chloride contaminated constructions and that applied to improve the corrosion resistance of the reinforcement of new structures expected to become contaminated are then underlined and discussed. The more recent applications of cathodic protection on carbonated concrete are also illustrated. The operating conditions (voltage and current applied), the throwing power, the protection conditions which avoid the risk of hydrogen embrittlement in prestressed structures are also discussed. Examples of cathodic protection and of cathodic prevention are also presented.