This review consists of three parts. The first part provides basic information about the physics and materials science of high-temperature superconductors for applications in magnet and energy technology. It addresses more than is required by a newcomer in this field, by putting together information from very different areas such as physical chemistry, materials science and condensed matter physics, which otherwise would have to be collected from various textbooks and original literature. In an introductory chapter potential applications are presented and the limitations of the materials are...