What is a “metamaterial”? In recent years, there has been a growing interest in the fabricated structures and composite materials that either mimic known material responses or qualitatively have new, physically realizable response functions that do not occur or may not be readily available in nature. The unconventional response functions of these metamaterials are often generated by artificially fabricated inclusions or inhomogeneities embedded in a host medium or connected to or embedded on a host surface. Exotic properties for such metamaterials have been predicted; many experiments have confirmed our basic understanding of many of them. The underlying interest in metamaterials is the potential to have the ability to engineer the electromagnetic and optical properties of materials for a variety of applications. The impact of metamaterials may be enormous: If one can tailor and manipulate the wave properties, significant decreases in the size and weight of components, devices, and systems along with enhancements in their performance appear to be realizable.