The photochemistry of aromatic compounds is classified into the similar categories adopted in the previous reviews in the series.^1–5  The photochemical properties of intramolecular photocyclization and cycloreversion of 1,2-di(hetero)arylethenes both in solutions and in solid and crystalline states and (E)/(Z) photoisomerization of azobenzenes are still interesting subjects, however this chapter does not deal with them. Dye (organic dyes, Ir and Ru complexes)-sensitized photoreactions have been widely reported by use of visible LED lights in view of green sustainable chemistry.^6–8  Stereoselective photoreactions in solid and crystalline states including cocrystals appeared in this period. In addition, it should be noteworthy that photoinduced transformation of organic molecules have been carried out using flow systems.^9,10  The advantages of this system are as follows: (1) the reaction temperature and the reaction time (the residence time) can be readily controlled. (2) The reaction vessel is safely small. (3) Heterogeneous reactions efficiently occur by use of alternating flow (slug flow) systems. (4) The formation of secondary products and/or by-products is suppressed because of the short residence time of the primary products. (5) Solvent and reagent wastes are minimized and environmentally friendly.