Photoresponsive processes are ubiquitous in nature and in artificial synthesis and catalysis. In 2013, Nelson and Bielawski wrote a perspective 72 that differentiated photoswitchable catalysis from photocatalysis and photoactivated catalysis by defining photoswitchable catalysis as a system involving a catalytically active species that can undergo a reversible photochemical transformation, which consequently changes its intrinsic catalytic properties. Here, we will focus on photoswitchable catalysis. In photoswitchable catalysis, photochromic functionalities such as azobenzenes, which can undergo E-Z isomerization, and diarylethenes, which can undergo photo-induced ring-closing, are commonly employed. When describing such systems several key descriptors are used: irradiation time, wavelength, the photostationary state equilibrium ratio between the two isomers, and the relative rates of the two states of the catalyst, kfast/kslow.