This text unifies the concepts of information, codes and cryptography as first studied by Shannon in his seminal papers on communication and secrecy systems. The first five chapters cover the fundamental ideas of information theory, compact encoding of messages and the theory of error-correcting codes. After a discussion of mathematical models of English, there is an introduction to the classical Shannon model of cryptography. This is followed by a brief survey of those aspects of computational complexity needed for an understanding of modern cryptographic methods and the recent advances in public key cryptography, password systems and authentication techniques. Because the aim of the text is to make this exciting branch of modern applied mathematics available to readers with a variety of interests and backgrounds, the mathematical prerequisites have been kept to an absolute minimum. Problems and solutions are included.