This book describes the status of research and development in the field of high-temperature superconductivity reached in the mid of the twenty-twenties. Starting from the milestones in the history of superconductivity, the main characteristics of the superconducting state are presented. Special physical properties of high-temperature superconductors are highlighted. Main classes of superconducting materials are introduced with the focus on high-temperature superconductors (cuprates and iron-based superconductors) and MgB2. Besides the material properties relevant for applications, the deposition of superconductor films and the manufacture of high-temperature superconductor wires are described. An outlook toward the future is included, covering potential applications of high-temperature superconductors in magnet technology and the electric power system.