Electrochemical Impedance Spectroscopy (EIS) has become an important research tool in Solid State Ionics. Some new developments are highlighted: new methods of automatic parameter extraction from impedance measurements are briefly discussed. The Kramers-Kronig data validation test presents another powerful tool in the impedance data analysis. The partial-fit and subsequent subtraction procedure, in combination with the K-K test, can lead to a more appropriate equivalent circuit model for the Complex Nonlinear Least Squares (CNLS)-fit routine. Very low frequency measurements ( < 1 mHz) are of significant interest for characterization of intercalation electrodes (anodes and cathodes) for novel high energy density batteries. The low frequency impedances can be obtained by simple means through a carefully conducted Fourier transform of the time domain relaxation data. Impedance analysis of thin film LixCoO2 cathodes shows that combination of a Finite Length Warburg (FLW) with a Finite Space Warburg (FSW) is viable model for the Li+ diffusion process. (C) 2004 Elsevier B.V. All rights reserved.