Studies of impedance characteristics have played an important role in both fundamental and applied electrochemistry for many years. With respect to the fact that reliable automatic measuring apparatuses operating in a quite large frequency range from 0.001 Hz to 1 MHz are commercially available, it may be expected that impedance measurements will remain popular among scientific and technical workers interested in electrochemistry. Naturally, this sort of work requires to understand the relevant theory. The present instructive guide is intended to serve this purpose. In preparing the second English edition, the text has been revised and amended where desirable, and two chapters dealing with diffusion in a limited space and a chapter on four-electrode systems were inserted. The author attempted to restrict, as far as possible, difficult mathematical sections and to provide the reader with bases on which he could further build. For those interested in the mathematics, a mathematical supplement was added for illustration, being not inevitable for understanding the matter. Besides the theory, the reader will also find many useful hints for experimental work.