This article describes the basic principles of the Electrochemical Impedance Spectroscopy, EIS, technique and its application in the characterization of electrode materials and electrochemical devices for energy conversion and storage. The concept of impedance and the necessary steps of impedance spectra analysis are explained. Basic relations for impedance of particular electro- chemical processes occurring in the system(s) are derived. Electrical equivalent circuits as impedance analogues for single-cell units of electrochemical (super)capacitors, galvanic cells (batteries) and fuel cells, as well as separate impedance components and parameters describing the system(s) are presented and discussed. The advantages and problems in application of the EIS technique for evaluation of operating states and possible failures of these electrochemical devices are described.