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.