Putative light-sensing proteins are ubiquitously encoded in the genomes of chemotrophic, non-photosynthetic bacteria. Surprisingly, these are not limited to UV-receptors: the metagenome of the chemotrophic prokaryotes encodes representatives of all known major families of photoreceptors. Insight into the mechanism of light-mediated signaling is relatively advanced, but most light-induced physiological and behavioral responses in chemotrophic bacteria are not well understood. In the current era of 'omics' studies, this knowledge gap could be closed rapidly. Here we review the state of the art in this field. Because light signals can be manipulated accurately, these photoreceptors might help provide a systems-level understanding of the cytology of bacteria.