Among the photochemical substances which are air pollutants, those which are known to have a strong toxicity towards plants include ozone, PAN, and nitrogen dioxide. Even though ozone is the main one of the photochemical substances, there still appears to be argument concerning the percentage it occupies among them. In California, 90% to 95% of the photochemical substances are ozone, while the rest consists of PAN and others. In Japan the problem has become serious only quite recently, but it has been said that the percentage occupied by ozone has a low value which is quite different from that of California. Little is known as yet about the composition of the photochemical substances in various sections of Japan. The historical background and the present situation concerning the occurrence of plant damage in Japan from photochemical substances is reviewed.