Quantum mechanics is the most successful theory for describing the micro-world of photons, atoms, and their aggregates. It is behind much of the successes of the modern technology. It has deep philosophical implications to the fundamental nature of material reality. A few decades ago it was also realized that it is connected to the computer science and information theory. With this understanding was born the new disciplines of quantum computing and quantum communication. The current book introduces the very exciting area of quantum communication which lies at the intersection of quantum mechanics, information theory, and atomic physics. The relevant concepts of these disciplines are explained and their implication for the task of unbreakably secure communication is elucidated. Mathematical formulation of various approaches are explained and attempt has been made to keep the exposition self-contained.