Publisher Summary Information is something that adds to people's knowledge. Information is not visible without some medium being a carrier. Data are the logical media often carried by some physical media such as a CD or a communication channel. Hence data can be viewed as a basic form of some factual information. This should be distinguished from other contrasting forms of information such as text, graphics, sound, and image. A large amount of data can then be organized and stored in short messages or long files. Information theory provides a good foundation for compression algorithms. By working on symbolic data, statistical models can be adopted for solving compression problems. Prefix codes, Kraft inequality, and entropy are useful tools in searching efficient codes and identifying the quality of compression algorithms. Symmetric and asymmetric compression models offer different performance and are useful for different types of sources. A good compression algorithm often relies upon effective identification of redundancies in the interested source.