LOV proteins are UVA/Blue light photoreceptors employing LOV (light, oxygen, voltage) domains as photosensory modules. They are wide spread among plants, fungi, bacteria and archaea. LOV domains are /β folds of ca. 110 amino acids and bear a fully oxidized FMN (flavin mononucleotide) chromophore, non covalently bound in the dark adapted state of the protein. Upon photoexcitation, FMN forms a covalent bond with a nearby cysteine (between the cys-SH group and position 4a of the flavin)during the decay of the FMN triplet state (Figure 1). According to its absorption maximum, the adduct is referred to as LOV390 (Figure 2) and is supposed to be the signaling state in vivo. In most LOV proteins, LOV390 thermally recovers to the dark-adapted state, LOV447. Signal propagation from the LOV core to effector domains or protein partners occur via the antiparallel β-scaffold and helical caps flanking the LOV core (figure 3). The peculiar photobiophysics of LOV proteins can be exploited for advanced biotechnological applications, such as fluorescence imaging, superresolution microscopy and optogenetics (figure 4).