We review the bacterial proteins related to the flavin-binding plant UVA/Blue-light sensors phototropins (phot), photoactivated adenylyl cyclase (PAC) and cryptochromes (Cry). Phot and PAC sense light via specialized domains, called respectively LOV (Light, Oxygen, Voltage) and BLUF (Blue Light sensing Using FAD). Genome digging reveals that about 16% of bacteria possess genes encoding for LOV, BLUF or/and Cry proteins. Up to now, their physiological role as photoreceptors has been established only for the BLUF proteins AppA in Rhodobacter sphaeroides and Slr1694 in Synechocystis sp. PCC 6803, but light-driven reactions have been demonstrated also for LOV proteins and detailed structural information is available for a cyanobacterial Cry. Bacterial LOV and BLUF proteins are highly modular and contain diverse catalytic functions (e.g. kinases, phosphodiesterase) associated to the photosensing domain, highlighting their involvement in various signal transduction pathways. This modularity can constitute a powerful tool (and a potential model for protein engineering) in understanding the modalities of interdomain communication in sensor proteins, given that signal transduction can be easily triggered by a light pulse. Thanks to the large spreading of flavin-based bacterial blue-light sensing proteins, phylogenetic analysis may provide important clues to understand how similar proteins have evolved in eukaryotic organisms. Furthermore, investigation of their physiological role is likely to provide us a more comprehensive view of how bacteria “see” their world.