Since light is an important environmental variable, many organisms have evolved signalling pathways that transmit and thereby translate this stimulus into various biochemical activities. Recently, new classes of blue light photoreceptors have been identified that use flavin based photosensors. The photosensor domains are coupled to an array of other domains, including kinases and transcription factors. We are studying proteins containing LOV and BLUF domains as photosensors using crystallographic, biochemical, and quantum chemical approaches. Together with spectroscopic data, their combination allows to understand on a molecular level how absorption of a blue light photon results in a specific structural change of the protein that triggers a secondary signal resulting in a biological event.