We live in an information society. This book describes the basic ideas of information theory, showing how we can transmit and store information as compactly as possible, how much information can be transmitted by a particular channel or network, and how security can be assured. It covers all the basic ideas of information theory and sets them in the context of current applications. These include Shannon's information measure, discrete and continuous information sources and information channels with or without memory, source and channel decoding, rate distortion theory, error correcting codes and the information theoretical approach to cryptology. Throughout the book the author has paid special attention to multiterminal or network information theory. This text will be of interest to advanced undergraduates and graduate students in electrical engineering, computer science, informatics, mathematics, physics and management sciences.