Metasurfaces for flat optics. Metasurfaces consisting of subwavelength optical antenna arrays have emerged as planar optical devices that enable many promising applications. Thesubwavelength antenna is designed to locally control the amplitude, phase, or polarization of light. Metasurfaces provide unprecedented opportunities to overcome the limitations of conventional lenses and have demonstrated promising applications, such as high-numerical aperture ultrathin lenses, high-resolution multicolor holograms, and optical skin cloaks. Metasurfaces have been developed for specifi c functionalities by exploiting new materials and design algorithms. The articles in this issue of MRS Bulletin provide a comprehensive understanding of metasurfaces and their novel applications. The cover shows the operating mechanism of a metasurface used as a lens (metalens) and a scanning electron microscope image of a particular metalens in the background. The metalens can focus light beams using phase control by the metasurface. See the technical theme that begins on p. 180.