A non-destructive estimation of the state-of-charge (SoC) of batteries facilitates optimum utilization of the battery for a given application, as well as evaluation of its state-of-health. Among various methods, a.c. impedance spectroscopy over a wide range of frequencies provides a variety of parameters which are functions of the SoC of a given battery. These parameters and their variation with SoC depend on the type of battery and on the experimental conditions. The impedance data of sealed commercial cells are rather tedious to analyze since the results are generally the combined parameters of both the positive and the negative electrodes. Separation of the parameters which correspond to the individual electrodes involves assumptions, the validity of which is open to question. Nevertheless, systematic linear and reproducible variations of measurable or computable impedance parameters with battery condition are useful indicators of the SoC in a non-destructive way. The present review consolidates the literature on the prediction of the SoC of batteries by means of a.c. impedance measurements. (C) 2000 Elsevier Science S.A. All rights reserved.