Optical metamaterials have been developed to control light using assemblies of electromagnetically interacting nanostructures (sometimes called meta-atoms), often exploiting their resonant behavior. The dimensions, material, topology of meta-atoms, layout of their assemblies, as well as the nature and mechanism of their interaction with light vary widely, and so do the targeted spectral response and functionalities. Super- and hyper-lenses for high-resolution imaging [1,2], sensing and filtering [3], signal processing and nonlinearity enhancement [4–6] have been demonstrated, to name but a few. Recently, the metamaterial approach has been simplified to metasurfaces, two-dimensional subwavelength arrangements of meta-atoms which replace the necessity of complex three-dimensional nanostructuring. Metasurfaces can control the local phase of reflected and transmitted light and, therefore, emulate light reflection, transmission, and diffraction of a 3D medium with a 2D structure of, generally, subwavelength thickness. In order to achieve a strong effect of meta-atoms on the incident light, resonant optical modes need to be engineered. This can be achieved with a multipolar response provided by either high-index dielectrics or plasmonic materials [7–11]. In the former, the electromagnetic field at the resonances 118occupies the volume of nanostructures, whereas in the latter they are localized at the interface between structures and the surrounding dielectric medium. The latter plasmonic metasurfaces are the focus of this chapter.