This book treats superconductivity from a theoretical phenomenological viewpoint and describes technological applications. The authors use a language common to physiscists and electrical engineers. After studying this book, the reader should be able to approach the more advanced literature on superconductivity and its applications. Structured as a series of quesitons and answers, the text will serve as a valuable tutorial for researchers and graduate students. The presentation is self-consistent, supplying information on the history of superconductivity and gas liquefaction, types of superconductors, their structures and the values of the main superconducting parameters, phenomenological theories of superconductivity, and wide-ranging applications.