An advantage of photoswitchable catalysis is that light is the only reagent required for switching, therefore, purification does not require removing excess reagents. Additionally, switching can be very fast and not limited by mass transport. Although it is not the main focus of this section, the utility of photoinduced energy or electron transfer to reversibly activate-deactivate polymerization is an important approach to use light as an external stimulus, as highlighted by pioneering works of Hawker 79 , Fors 39 , and Boyer. For example, Fors and coworkers 81 demonstrated how to alter the reactivity of a catalyst by changing the wavelengths of light. In their work, by using proper photocatalysts and a thiocarbonate chain transfer agent, cationic polymerization could be initiated by green light, while radical polymerization could be commenced by blue light. Photoswitchable catalysis also needs to overcome several hurdles; for example, a short irradiation time is required to obtain a high photostationary state isomer ratio, and the isomers need to demonstrate distinct activities. In terms of photon absorption, one precaution is to keep the concentration of the photoswitchable catalyst within the applicable range of Beer's law and to take into account the absorption of other ingredients such as substrates, cocatalysts, and solvents in the system. Notwithstanding all the challenges, photoswitchable catalysis provides a non-invasive method to achieve temporal control.