The measurement of current and voltage fluctuations on the same electrochemical cell allows the evaluation of a quantity R(n) known as noise resistance, which has been proposed as an indication of the corrosion resistance of the material under study. The theoretical basis for the relationship between R(n) and the electrode impedance Z is developed, taking into account the various measurement schemes currently in use. Parameters such as cell geometry, solution resistance, and electrodes with different kinetics are considered. It is shown that, in general, the modulus of the electrode impedance can be derived by measuring the power spectral densities (PSD) of the voltage and current noises. The circumstances in which R(n) is equal to the polarization resistance of the electrode are also discussed.