This book deals with the contemporary development of quantum theory from the point of view of quantum optics. The fundamentals of quantum theory are presented, with particular attention being given to the quantum theory of measurement. General coherent states are adopted as the most important theoretical tool for the description of the interaction of optical fields with matter. Quantum theory predicts various nonclassical phenomena, such as squeezing of vacuum fluctuations, photon antibunching, sub-Poisson photon statistics, collapses and revivals of atomic motion, etc. These quantum effects can be observed, together with violation of various classical and Bell's inequalities, using photon interference and correlation techniques. This is demonstrated by experiments involving single-photon and two-photon interferences, squeezed and sub-Poisson light and twin photons produced by frequency down conversion. All these results are in agreement with quantum theory and they confirm the wave--particle duality of a photon and the `nonlocal' features of quantum theory. For research workers in quantum mechanics, quantum theory, quantum optics and electronics, optical communications, optoelectronics, photonics and nonlinear optics, optical processing and computing.