The topic of information theoretic security is introduced and the principal results in this area are reviewed. The basic wire-tap channel model is considered first, and then several specific types of wire-tap channels are considered, including Gaussian, multi-input multi-output (MIMO), compound, and feedback wire-tap channels, as well as the wire-tap channel with side information. Practical code design techniques to achieve secrecy for wire-tap channels are also introduced. The wire-tap formalism is then extended to the basic channels of multi-user networks, including broadcast channels, multiple-access channels (MACs), interference channels, relay channels and two-way channels. For all of these models, results on the fundamental communication limits under secrecy constraints and corresponding coding schemes are reviewed. Furthermore, several related topics including key agreement through common randomness, secure network coding, authentication, cross-layer design, and secure source coding are discussed.