The growing number of sequenced prokaryotic genomes reveals a wide distribution of open reading frames (ORFs) that putatively encode for red- and blue light sensing photoreceptors. They comprise the bilin-binding phytochromes and the flavin-binding cryptochromes, LOV and BLUF proteins, indicating that about 1/4 of bacteria do possess at least one of these photosensory proteins. The distribution of red- and blue-light sensors among different prokaryotic phyla and classes, and their functional activity as light-switched systems are the subject of this perspective. These photoreceptors were originally found in plants by following the associated physiological responses induced by the respective spectral irradiation. Genome-based approaches now require the assignment of a photochemical/physiological function to the heterologously expressed gene product. Database searches demonstrate in some cases several genes of one category in a certain prokaryot, indicating the presence of more than one type of red- or blue-light sensing properties, but also show a combination of proteins with both spectral sensitivities. Another interesting feature now “comes into light”: according to their nature as biological sensors, these photoreceptors are equipped with signalling domains, initiating a cellular response, thereby constituting modular systems switchable by light. It is seen that many of these signalling domains, now found together with light-inducible sensing domains, were already described for other stimuli, e.g., osmo-regulation, oxygen, hydrogen, chemicals, or pH. In some cases, the same type of signalling domain can be found in a red- or a blue-light sensing photoreceptor. Following the characterization of their photochemistry, for several of these bacterial photoreceptors physiological functions are now assigned.