[2 + 2] Photocyclization and photoreversion in polymer chemistry is briefly described as well as the properties and functions of the resulting polymers. [2 + 2] Photocyclization is successfully applied in 1) crosslinking between the side chains and between the main chains of polymers in solid state, 2) polymerization of conjugated diolefins in crystalline state, and 3) solution polymerization of bisolefins These reactions afford the formation of image. Polymers having cyclobutanes in main chain can be also prepared by the use of the photocyclodimers of olefins. Among them, anti head-to-head coumarin dimer is susceptible to nucleophilic attack to give high-molecular-weight polyamides by the reaction with diamines. Some of the polyamides show an unique photochemical behavior and have chiral recognition ability