This book begins by introducing the basic concepts of impedance to non-specialist readers who 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, with many simulated examples, before moving on to develop key ideas relating to the application of impedance spectroscopy to electrochemical systems. Circuit elements used to model electron transfer, double layer capacitance and diffusion are described in detail, along with Kramers-Kronig testing of experimental data. After explaining how potentiostats and frequency response analysers work, the book analyses a wealth of real-life experimental data that have been obtained either during the annual EIS courses in Bath or in research carried out in the laboratories of the author and his colleagues in Bath. Topics covered include not only conventional electrochemical systems such as the rotating disc electrode and ultramicroelectrode but also solar cells, using data obtained for dye-sensitized solar cells and perovskite solar cells for illustration. The application of frequency-resolved methods in optical spectroscopy is illustrated with results from polyaniline electrochromic windows and hematite photoelectrodes, and finally the last two chapters introduce techniques based on modulation of light intensity rather than voltage or current -- examples are intensity modulated photocurrent /photovoltage spectroscopy (IMPS/IMVS). The book concludes with worked answers for the problems set out in earlier chapters.