High-flux optical systems (HFOSs) are optical concentrators used to increase the radiative flux of the natural terrestrial solar irradiation. High radiative flux concentration leads to high energy density in solar receivers which allow to obtain high temperatures. In solar thermochemical applications, the high-temperature heat drives endothermic thermochemical reactions. HFOSs have been deployed for research and development of solar thermochemical devices and systems, from solar reacting media to solar reactors. Here, we review the designs and characteristics of HFOSs as well as challenges and opportunities in the area of high-flux optical systems for solar thermochemical applications.