Semiconductor-based metasurfaces are an emergent area of photonics that have surged in the past decade.1–3 Exciting applications of these novel materials, such as wavefront engineering,4–7 sensing,8–11 and nonlinear optics,12,13 are extensively covered throughout this book. The promising features of semiconductor materials are their transparency below the electronic bandgap, pronounced optical nonlinearities, and charge carrier dynamics that lead to extraordinary opportunities in ultrafast photonics. The goal of this chapter is to provide the reader with an overview of ultrafast semiconductor-based metasurfaces and their applications. The structure of this chapter is as follows: Section 6.2 describes the ultrafast processes taking place after an ultrashort laser pulse excites a semiconducting material; Sections 6.3 and 6.4 review ultrafast phenomena in Mie-resonant nanostructures and metasurfaces 182that utilize silicon and gallium arsenide, respectively; Section 6.5 is devoted to photon acceleration in semiconductor metasurfaces; Section 6.6 covers the ideas of how semiconductor-based metasurfaces can be utilized for shaping of ultrafast laser pulses; Section 6.7 concludes the chapter and briefly outlines the emerging directions within ultrafast photonics of semiconductor metasurfaces.