Fundamentally, a filter is a circuit whose purpose is to separate one or more signals from one or more others. Filtering allows you to select a particular radio station from the hundreds that are simultaneously present at your radio’s antenna. Filtering is used to process radar and sonar echoes, to clean up old recordings, and in a host of other applications. Filters are present in virtually all electronic systems for communication, control, instrumentation, echolocation (radar, sonar, infrared), image processing, and (of course) signal processing. It is hard to think of a significant electronic system that does not involve filtering.