This book begins by introducing the basic concepts of impedance to non-specialist readers, who may have only an elementary knowledge of physics and mathematics. Mathematical concepts are explained clearly at appropriate points in a series of Theory Notes. Subsequent chapters cover RCL (resistor, capacitor, inductor) circuits before developing the key ideas behind the application of impedance spectroscopy to electrochemical systems. Circuit elements used to model electron transfer, double-layer charging and diffusion are described in detail, along with Kramers-Kronig testing of experimental data. The book explains how potentiostats and frequency-response analyzers work and evaluates a wealth of experimental data obtained either during the annual Bath impedance courses or in the laboratories of the author and his colleagues. Topics covered include not only conventional electrochemical systems, such as the rotating disc electrode and ultramicroelectrodes, but also unconventional solar cells and the application of frequency-resolved techniques in spectroelectrochemistry. Finally, the last two chapters introduce techniques based on modulation of light intensity rather than voltage or current. The book concludes with worked answers to the problems set out in earlier chapters.