This chapter describes the roles and basic concepts of analog filters, theirapplications in electrical engineering, and certain fundamental ideas andterminology associated with them.The term filter is used in many different ways in electrical engineering.An algorithm in a computer program that makes a decision on whichcommands and how certain commands are executed performs a filteringfunction. A decision technique that estimates the input signal from a setof signals and noise is known as optimal filtering. In analog and digitalsignal processing, filters eliminate or greatly attenuate the unwantedportion of an input signal. These analog and digital filtering processesmay be performed in real-time or in off-line situations.