An appreciable amount of the light that hits the surface of the earth (about 40% of the incident energy) is blue light of the wavelengths 400-500 nm. This light can be used either as a source of energy-as algae and higher plants use it with their accessory pigments in photosynthesis-or as a source of information in controlling or initiating certain physiological processes. Such blue-lightregulated responses include phototropism and phototaxis, suppression or shifts of circadian rhythms, induction and stimulation of spore-formation in certain fungi, in some cases induction of protein synthesis and activation of enzymes, and initiation of carotenoid synthesis. In this article, I will discuss the photoreceptors for some blue-light-controlled reactions, excluding the process of vision where blue light is not a specific source of information even if one absorbance maximum of the cone-cell pigments of human color vision is at 447 nm.