This chapter discusses the fundamentals of channel protection that lie beneath the error control techniques used to communicate multimedia over the Internet and wireless networks. The goal of a classical communication system is to transfer the data generated by an information source efficiently and reliably over a noisy channel. The basic components of a digital communication system are a source encoder, channel encoder, modulator, demodulator, channel decoder, and source decoder. The source encoder removes the redundancy in the digital data produced by the information source and outputs an information sequence. If the information source is analog, its output must be digitized before it is processed by the source encoder. The channel encoder adds redundancy to the information sequence so that channel errors can be detected or corrected. The output of the channel encoder is a finite sequence of symbols called a "channel codeword.". 2007 Elsevier Inc. All rights reserved.