System identiœcation techniques are applied to estimate from experimental data the coe¥cients of the impedance transfer function, Z(s). še current, I(s), is the input signal and the output signal is the voltage, V(s). še experimental data used to perform the estimation are the transient I-V (currentvoltage) response of the fuel cell to certain electrical stimuli, such as CI, current pulse and pseudo random binary sequence (PRBS) signal, and so on. šese stimuli are usually generated, thereby forcing changes in the electrical load applied to the fuel cell. Dižerent experimental procedures have been devised. Some of them ažect the fuel cell operation (e.g., sequences of CIs), while others require performing small changes in the electric load and consequently do not interfere appreciably with the normal operation of the fuel cell.