In numerous applications of signal processing and communications we are faced with the necessity to remove noise and distortion from the signals. These phenomena are due to time-varying physical processes, which sometimes are unknown. One of these situations is during the transmission of a signal (message) from one point to another. The medium (wires, fibers, microwave beam, etc.), which is known as the channel, introduces noise and distortion due to the variations of its properties. These variations may be slow varying or fast varying. Since most of the time the variations are unknown, it is the use of adaptive filtering that diminishes and sometimes completely eliminates the signal distortion.