Impedance spectroscopy has been used to investigate various electrochemical devices and electrode reactions. The principle of impedance spectroscopy is explained in this article. The relationships between simple circuits and the impedance spectra are revealed using Nyquist and bode plots. Impedance spectra of an electrode/electrolyte interface are obtained as a semicircle on the Nyquist plot. The equivalent circuit for this interface involves the charge transfer resistance and the interfacial capacitance in parallel, and comparison of this circuit with the impedance spectra provides the structural and physical information concerning the interface.