Since light is an important environmental variable, many organisms have evolved signaling pathways that not only sense, but also transmit and thereby translate this stimulus into various biochemical activities. We study a variety of photoreceptor proteins that use different chromophors for light absorption. A major focus is on blue-light absorbing flavin-based photosensors that are coupled to an array of other domains such as kinases and transcription factors. To characterize the light-induced structural changes in the sensor domain and to understand how they activate the regulated output domain, we use biochemical and biophysical approaches including various structural methods, such as hydrogen deuterium exchange coupled to mass spectrometry (HDX-MS), static and time-resolved X-ray diffraction and solution scattering. This is supported by computational studies performed in Tatiana Domratcheva’s group. Together with spectroscopic data, this allows us to understand on a molecular level how absorption of a photon results in a specific structural change of the protein that triggers a secondary signal resulting in a biological event.