The purpose of this book is to present the concept of impedance, impedance of electrical and electrochemical systems, its limitations, and certain applications. The available books on EIS were written either by physicists or engineers, and I wanted to present it from the chemist’s point of view. Some knowledge of electrochemistry is necessary to understand the developments of kinetic equations. I hope that it will be useful to students who are just starting to use this technique and to others already using it in their research. The book contains theory and applications, numerical examples shown in the text, and exercises with full solutions on the Internet. First, electrical circuits containing resistances only are presented, followed by circuits containing R, C, and L elements in transient and ac conditions. To understand the concept of impedance, the notions of Laplace and Fourier transforms are presented and must be understood thoroughly. In this chapter, impedance plots are also presented, along with several examples for various circuits. Next, methods for determining impedances, including fast Fourier transform-based techniques, are discussed.