Frequency selective surfaces (FSS) have been the subject of great interest for more than three decades. Typically, an FSS consists of one or more grids of two-dimensional, periodically arranged array elements, which may be etched on or imbedded in one or multiple layers of dielectric laminates. FSSs have many applications as filtering devices in microwave and higher frequency bands. This introduction to frequency selective surfaces (FSS) covers both theory and applications. By way of example, it includes the computer code for one specific FSS design.