Supercritical fluid, which is defined as a state of fluid over the critical temperature and pressure, has high potential as a reaction and separation solvent. The development of chemical recycling of waste plastics by decomposition reactions in subcritical and supercritical fluids is reviewed including fundamental investigation and commercialization. Decomposition reactions proceed rapidly and selectively in supercritical fluids compared to conventional processes. Condensation polymerization plastics such as PET and nylon are relatively easily depolymerized to their monomers in supercritical water or alcohols. Crosslinked polymer can be recycled by selective decrosslinking reactions in supercritical fluid without severe decomposition of the backbone chains. Fiber reinforced plastics can also be recycled by depolymerization of the resin component to obtain recovered fibers and monomers. Pilot scale or commercial scale plants using subcritical and supercritical fluids have been developed for plastics recycling.