Communications, the transmission of information from a sender to a receiver (destination), is fundamentally a random experiment. The sender selects one of a number of possible messages which is presented (transmitted) to the receiver. The receiver has no knowledge about which message is chosen by the sender; however, this message is chosen from a set of possible messages known to both the sender and the receiver. In transition the signal carrying the message is distorted, mainly by channel noise. This noise in a communications link is a fundamental property of the link. Without noise, the concept of communications would not exist, since it would be possible to move arbitrary amounts of information virtually instantaneously. For example, if we could transmit a chosen voltage signal without noise, we could transmit an exact real number (the voltage level), whose infinite expansion into binary digits would give us an unlimited amount of data to transport. Noise then, in a sense, prevents information from being omnipresent, and it makes possible the modern concepts of communications. Since noise is a random process, it should not be surprising that communication theory draws heavily from probability theory.