The degradation of polymeric materials with chemical reaction in which the molecular chains is cut is called corrosion. In this paper the behavior, the forms and mechanisms of corrosion of resins and GFRP (glass fiber reinforced plastics) under several aqueous solutions are reviewed based on our experiments. Three types of corrosion exist: the surface reaction type, the corroded layer forming type and the penetration type. Each form was determined by the chemical structures of the resin, the reactivity between resins and environments, and also the diffusivity of environmental media into the resins. The concept of the corrosion rate of metals could be applied even in plastics and fiber reinforced plastics for each corrosion form.