The use of various impedance spectroscopies, including electrochemical impedance spectroscopy (EIS), photoelectrochemical impedance spectroscopy (PEIS), and those defined using various electrochemical, hydrodynamical, and mechanical conjugate variables, for studying physico-electrochemical systems is briefly reviewed with emphasis on identifying and defining a common theoretical basis. In particular, we explore the concepts of "conjugate properties" and "reciprocity", the applicability of the Kramers-Kronig transforms, and the distinction between an impedance and a transfer function. We argue that the interpretation of impedance data in terms of electrical analogs and in terms of classical linear systems theory (I,ST) is possibly too restrictive, and that a more general basis is required to accommodate the broad range of "impedance spectroscopies" that are now being developed.