Photosystem II is vulnerable to various environmental stresses. Especially, light and heat stress cause serious damage to the reaction-center binding protein D1 in photosystem II. Once damaged, the D1 protein is efficiently digested by specific proteases and is removed from photosystem II. Finally, a newly synthesized D1 protein is incorporated into photosystem II and photosystem II is repaired. In the present study, we showed that there are various strategies in photosystem II to avoid the stresses and also to tolerate the stresses. In the stroma of chloroplasts, there is an unknown proteolytic enzyme(s) which digests the cross-linked products of the D1 protein generated by excessive illumination of photosystem II. Within the thylakoid membranes, several proteases including the well known FtsH and DegP may function as the enzymes recognizing the damaged D1 protein. In the lumen of the thylakoids, extrinsic proteins of photosystem II, PsbO, P and Q stabilize photosystem II and probably function as scavengers of hydroxyl radicals generated in photosystem II under light stress. Using these various means, photosystem II can survive the hazardous environmental stresses.