Concrete protects the steel embedded until carbonation front or the chloride threshold reaches the rebar. Then a corrosion process develops. In present paper the corrosion process is analyzed from its onset a definition of which is given. The techniques for its measurement are described and also are given a model for the corrosion propagation period as well as some alternatives to assume a value of the corrosion rate, even based in the values of the concrete resistivity. Finally, the functions of relation between the corrosion and the structural consequences are commented.