This monograph, by now a standard reference in the field, provides a complete overview of quantum entropy theory, from the early days of quantum mechanics up to the most recent developments. It employs mathematical formalism based on operator theory and functional analysis with a goal of creating a comprehensive account of the different entropy concepts, their interrelations and use. Among the ideas discussed are entropy of a quantum state, relative entropy, mutual entropy, entropy of a coarse graining and dynamical entropy. Numerous fundamental properties of quantum information measurement are developed, including the von Neumann entropy of a statistical operator and its limiting normalized version, the entropy rate. Use of quantum-entropy quantities is made in perturbation theory, central limit theorems, thermodynamics of spin systems, entropic uncertainty relations, and optical communication. This new softcover corrected reprint contains summaries of recent developments added to the ends of the chapters.