Plants and animals use blue light photoreceptors to respond to environmental conditions. Examples are the growth of plants towards the light and the setting of the daily rhythm in insects. Most blue light photoreceptors use derivates of riboflavin (vitamin B2) as chromophore. Three classes of flavin-containing receptors have been identified in the recent past: phototropins, cryptochromes, and sensors of blue light using FAD (BLUF). The basic mechanisms differ from those of all other photosensory proteins such as the retinal in our eyes. We are investigating the signal transfer inside these proteins directly after light absorption