It is now recognized that environmentally occurring halogenated compounds are derived not only from anthropogenic sources but also from biogenic and abiogenic natural processes. Thus, halogens take part in a complex biogeochemical cycle, in which there may be a number of mechanisms for microbially mediated transformation of halogenated compounds. Since most of the (poly)halogenated compounds are highly toxic and persistent, apart from their origins, the study of natural attenuation and biotransformation of organohalides is a subject of major concern in environmental science and technology. Many halogenated compounds are reductively dehalogenated by certain anaerobic microorganisms.  The most important feature of reductive dehalogenation is the utilization of halogenated compounds as terminal electron acceptors in an anaerobic respiratory process coupled to ATP production, i.e., the energy metabolism called (de)halorespiration. A wide variety of anaerobic dehalogenating microorganisms, including a unique group of halorespirers, “Dehalococcoides”, have been isolated and characterized from phylogenetic, physiologic, and genetic points of view. In recent years, new perspectives of metabolic reductive dehalogenation have turned up based on whole genome information about halorespiring bacteria and developments in molecular microbial ecology. This review article overviews current knowledge on the biodiversity of anaerobic dehalogenating microorganisms with special emphasis on the molecular biology and ecophysiology of “Dehalococcoides” and its mixed populations exhibiting reductive dehalogenation. The application of mechanisms of reductive dehalogenation to bioremediation of organohalide pollution is also discussed.