This book highlights how superconductor detectors that replace conventional detectors, such as semiconductor detectors and secondary electron multipliers, can overcome the fundamental limitations of certain analytical instruments. As an educational guide to students entering this field, this book begins with the history of superconductor detectors and the fundamentals on detector operation, categorizing two detection schemes of thermal detection and quantum detection. For experts, the book offers a review on the oldest and latest endeavors. Comparison between the detector physics of superconductor detectors and that of semiconductor detectors reveals unsolved issues such as quasiparticle excitation, Fano factor, average energy consumed for generating carriers, phonon loss, and spatial inhomogeneity. Instrument developers will be stimulated by the comparison of performance figures of different superconductor detector types.  The novel measurement data included will contribute to a wide range of fields, such as dentistry, molecular biology, energy-saving society, planetary science, and prebiotic organic molecules in space. This book is correspondingly suitable for a broad audience in the detector research community, seeking to deepen their understanding of detector physics and its applications.