  Metasurfaces enable flat optical elements by leveraging optical resonances in
metallic or dielectric nanoparticles to obtain accurate control over the
amplitude and phase of the scattered light. While highly efficient, these
resonances are static and difficult to tune actively. Exciton resonances in
atomically thin 2D semiconductors provide a novel and uniquely strong resonant
light-matter interaction, which presents a new opportunity for optical
metasurfaces. Their resonant properties are intrinsic to the band structure of
the material and do not rely on nanoscale patterns and are highly tunable using
external stimuli. In this tutorial, we present the role that excitons
resonances can play for atomically-thin optics. We describe the essentials of
metasurface physics, provide a background on exciton physics, as well as a
comprehensive overview of excitonic materials. Excitons demonstrate to provide
new degrees of freedom and enhanced light-matter interactions in hybrid
metasurfaces through coupling with metallic and dielectric metasurfaces. Using
the high sensitivity of excitons to the medium's electron density, the first
demonstrations of electrically-tunable nanophotonic devices and atomically-thin
optical elements are also discussed. The future of excitons in metasurfaces
looks promising, while the main challenge lies in large-area growth and precise
integration of high-quality materials.
