This two-volume handbook offers a comprehensive and coordinated presentation of SQUIDs (Superconducting Quantum Interference Devices), including device fundamentals, design, technology, system construction and multiple applications. It thus bridges the gap between fundamentals and applications, and will be a valuable textbook reference for graduates and for professionals engaged in SQUID research and engineering. It will equally be of use to specialists in multiple fields of practical SQUID applications, from human brain research and heart diagnostics, via airplane and nuclear plant testing to prospecting for oil, minerals and buried ordnance. The first volume presents the theory of SQUIDs, their fabrication from low- and high-temperature superconductors, the necessary readout electronics, and the design and performance of practical direct current and radio-frequency SQUIDs. It concludes with an overview of the most important SQUID system issues. In addition, an appendix summarizes the foundations of superconductivity that are necessary to understand SQUIDs, while a glossary and tables of units and constants are also included. The second volume is devoted to applications.