The usefulness of EIS lies in the ability to distinguish the dielectric and electric properties of individual contributions of components under investigation. This technique is a non-destructive and so can provide time dependent quantitative and time-dependant information about the system. The mathematical approach of electrochemical impedance data is based on the Ohm’s law, i.e. on the linear interdependency between potential perturbation and current response or vice versa. EIS data is commonly analyzed by fitting it to an equivalent electrical circuit model consisting of passive elements that do not generate current or potential such as resistors (R), capacitors (C), and inductors (L). To be useful, the elements in the model should have a physical meaning in the physical electrochemistry of the system. The interpretation of EIS data is built upon reaction model (or equivalent circuit) consisting on passive elements connected in some order. With the equations of the model it is then possible to calculate the electrochemical impedance as a function of frequency and check conformity experimental data to theoretical model using commercial software.  The applicability of EIS for the characterisation of semiconductors (oxide films), insulating films, EIS responses during cathodic iron-group metals and alloys films deposition and their corrosion, for optimization pulse current parameters for electrodeposition is discussed. Here an example of the study of WO3 films electroforming in oxalic acid and their characterization is provided in Fig. 1.