Light, oxygen and voltage (LOV) and blue-light sensing using FAD (BLUF) domains constitute two classes of recently discovered photoreceptors that are found in plants, algae, fungi and bacteria. They are sensitive to blue light through a non-covalently bound flavin chromophore. LOV domains belong to the superfamily of PAS (PER/Arnt/Sim) sensor and regulatory domains, whereas BLUF domains make up a distinct family that shows no relationship to other sensor proteins. LOV domains undergo a photocycle involving light-driven covalent adduct formation between a conserved cysteine residue and the C(4a) atom of the flavin cofactor. In BLUF domains, light absorption initiates a photocycle that most likely involves a hydrogen-bond rearrangement between the flavin cofactor and the residues lining the flavin-binding pocket. In this chapter, an overview is given of the insights gained from biophysical studies into the functional properties of these novel photoreceptor proteins. The three-dimensional structures, steady-state and time-resolved spectroscopy and photocycles of LOV and BLUF domains are reviewed. Their photophysics and photochemistry are discussed in terms of proposed reaction mechanisms. Finally, the light-induced conformational changes that are required for the signaling function of these photoreceptor domains are considered.