Electrochemical impedance spectroscopy is a powerful and general technique suitable for in-situ characterization of electrochemical systems. The fundamental concepts of impedance spectroscopy are presented
in the context of a hierarchy of electrochemical methods. As, in theory, time-domain and frequency-domain transient techniques provide the same information concerning the system under study, the selection between the two must be based on the frequency range that can be sampled and on the associated error structure. In the context of error structure, the advantages of impedance spectroscopy are that the integration at a single frequency over many cycles greatly reduces stochastic errors and that the measurement of a complex impedance allows use of the Kramers-Kronig relations to identify experimental bias errors. Keywords: corrosion, electrochemical methods, instrumentation, transient techniques