This book describes the application of high-temperature superconducting materials to microwave devices and systems. It deals with the fundamentals of the interaction between microwaves and superconductors, and includes a basic description of how microwave devices can be constructed using these materials. Since the discovery of high-temperature superconductors in 1986 there has been an enormous effort worldwide to develop and characterize these materials. Work on applications has proceeded more slowly however. Nevertheless, commercial applications are now beginning to be possible, including use in passive microwave devices. The advantages of using high-temperature superconductors in these devices is carefully described by the author, enabling scientists and engineers to form a complete understanding of the subject. The rest of the book is devoted to examples of superconducting microwave filters, antennas and systems. The examples chosen relate not only to what can be achieved at present, but indicate the trends for future research and what may be expected for superconducting devices in the future.