Steady-state techniques are very limited when they are used for analyzing corrosion processes for uniform or for localized phenomena. Non-steady-state techniques give valuable information concerning these processes. The measurement of the electrochemical impedance and the analysis of the electrochemical noise are particularly fruitful. In this paper, examples are given in the case of active and inhibited corrosion of iron, corrosion of coated metal, anodized aluminum alloys, and stainless steel. The principles of measurement techniques and analysis are also described.