This book discusses fundamental problems in quantum physics, with emphasis on quantum coherence and decoherence. Papers covering the wide range of quantum physics are included: atom optics, quantum optics, quantum computing, quantum information, cryptography, macroscopic quantum phenomena, mesoscopic physics, physics of precise measurements, and fundamental problems in quantum physics. The book will serve not only as a good introduction to quantum coherence and decoherence for newcomers in this field, but also as a reference for experts. Contents: Quantum Computing: Decoherence and Dephasing in Spin-Based Solid State Quantum Computers (S Das Sarma et al.); Quantum-State Manipulations in a Cooper-Pair Box (Y Nakamura et al.); Quantum State Engineering and Josephson Junctions: Charge and Flux Detectors (Yu Makhlin et al.); Quantum Information, Quantum Teleportation, and Entanglement: High-Fidelity Experimental Quantum Teleportation and Entanglement Swapping (A Zeilinger et al.); Experimental Realization of Continuous-Variable Teleportation (A Furusawa); Quantum Optics: Entanglement Manipulation with Atoms and Photons in a Cavity (S Haroche); Generation of Single Photons and Entangled Photon Pairs from a Quantum Dot (Y Yamamoto et al.); Twin Photon Beams for Single Photon Generation (S Takeuchi); Bose-Einstein Condensation and Atom Interferometry: Quantized Vortices in a Bose-Einstein Condensate (J Dalibard et al.); Vortex Excitations in a Bose-Einstein Condensate (S Inouye et al.); Mesoscopic Magnets: Environmental Effects on Quantum Reversal of Mesoscopic Spins (B Barbara et al.); Resistance of Geometrically Confined Magnetic Domain Wall (T Ono et al.); Single Electronics and Superconductors: A Single-Photon Detector in the Far-Infrared Range (O Astafiev et al.); Nanoscale Physics and Atomics: Quantized Conductance of Gold Nanowire Studied by UHV-Electron Microscope with STM (K Takayanagi); Quantum Transport: Quantum Transport in Two-Dimensional Electron Gas in Ultra-Short Period Lateral Superlattices (Y Iye et al.); Enhanced Tunnel Magnetoresistance in Ferromagnetic Single Electron Transistor (R Matsuda et al.); Precise Measurements: Oscillation Phenomena in High Energy Physics: CP Violation in B-Meson Decays and Long Baseline Neutrino Oscillation (K Nakamura); Dynamic Observation of Vortices in High-T c Superconductors (A Tonomura); Precision Optical Frequency Metrology Using Pulsed Lasers (Th Udem et al.); Interferometric Gravitational Wave Detector in Japan (N Mio); Fundamental Problems in Quantum Physics: Quantum Information Aspects of Black Hole (A Hosoya); and other papers. Readership: Undergraduates, graduate students and researchers in quantum physics, atomic physics and optics.