Real-time and off-line processing of continuous-time signals by digital means digital signal processing (DSP) has become a viable processing mode over analog means for several reasons, some of which are digital signal processors, microcontrollers, and microprocessors. These are inexpensive, programmable, reproducible, and consume low power, and have computing speeds suitable for signals with bandwidths beyond base band video, and can operate in extreme environments. Some broad application areas of DSP are automotive industry, consumer electronics, communication systems, and medical systems. The chapter considers the Fourier series (FS) concept for continuous-time periodic signals. A discrete-time system is said to be bounded-input-bounded-output stable if for a bounded input the output is bounded. The concept of a FS was developed as the minimum mean square error solution for the problem of approximating a continuous-time periodic signal with a linear combination of sinusoidal functions.