Because the quantum private communication has currently become a practical reality with products available commercially, it is important to focus not only on the theoretical topics but also on the practical issues. Accordingly, this book arranges the contents from pure theoretical descriptions to practical applications. To reach this aim, a broad range of materials are covered in this book, including how to protect confidentiality and authentication of the private communication using quantum tools and typical techniques for practical applications of quantum private communication in fiber telecommunication systems, wireless optical communication (including satellite communication), IP networks, and mobile communication systems, etc.