© 2013 Springer-Verlag Berlin Heidelberg. All rights are reserved. Luminous bacteria are those bacteria that carry the lux genes, genes that code for proteins involved in light production. Many luminous bacteria emit light at high, easily visible levels in laboratory culture and in nature, and the phenomenon of light emission has generated interest in these bacteria for over 125 years. Luminous bacteria are especially common in ocean environments where they colonize a variety of habitats, but some species are found in brackish, freshwater, and terrestrial environments. This chapter, which begins with an historical perspective, summarizes current understanding of the biochemistry and genetics of bacterial light emission, the taxonomy and phylogenetics of light-emitting bacteria, the evolutionary origins and hypothesized physiological and ecological functions of bacterial luminescence, the distributions and activities of these bacteria in nature, their symbiotic interactions with animals and especially with marine fishes, and the quorum sensing regulatory circuitry controlling light production at the operon level. This chapter concludes with information on the isolation, cultivation