The members of this genus respire halogenated compounds with an array of carbon backbones of biogenic and anthropogenic origin. Of particular importance for bioremediation are Dehalococcoides mccartyi strains that reduce the widespread soil and groundwater contaminants perchloroethene, trichloroethene, and the daughter chlorinated products, cisdichloroethene and vinyl chloride to the nontoxic, non-chlorinated end product, ethene. These strains couple the reductive dechlorination of these chlorinated ethenes to growth using H2 as electron donor and acetate as carbon source. A range of contaminated environments has served as microbial inocula for Dehalococcoides enrichment ICI 182780 Estrogen Receptor inhibitor cultures throughout the two decades of research on reductive dechlorination. Table 1 contains a compilation of cultures employed in fundamental studies and in bioaugmentation research/applications for PCE or TCE dechlorination. Development of these enrichment cultures is a lengthy process as the enrichments must be actively fed and transferred to maintain the desired biological activity. Careful consideration is given to any environmental sample before pursuing this labor- and time-intensive work. Often crucial in deciding to i) develop novel reductively dechlorinating enrichment cultures, ii) biostimulate, and iii) bioaugment a contaminated site is evidence of reductive dechlorination to VC and ethene. Hence, VC and ethene are measured either in laboratory microcosm experiments or directly, during evaluation of contaminated sites. This information is not always reported per se; however, for the majority of the enrichment cultures in Table 1, there was evidence of desired biological activity through one or both assessment methods. Observations on the presence of Dehalococcoides and the stalling of PCE/TCE dechlorination at cis-DCE or VC are also common. This puzzling outcome has been reported in soil and sediment microcosm studies and in bench-scale bioremediation scenarios, as well as at contaminated sites undergoing bioremediation. Whereas some of the abovementioned works did not put forth an explanation on the inability to achieve dechlorination of cis-DCE or VC, the most commonly proposed reason was the absence of Dehalococcoides mccartyi strains with DCEand VC-respiring metabolic capabilities. Nonetheless, this unpredicted outcome was also noted even when the identified Dehalococcoides mccartyi genes vcrA and bvcA coding for VC reductive dehalogenase enzymes were detected. Yet, neither VC reduction nor increases in Dehalococcoides mccartyi occurred in microcosms biostimulated with a fermentable substrate as the precursor for H2 and acetate. We hypothesize that, often, the discrepancy between the expected and the observed activities of Dehalococcoides is not due to their metabolic potential; instead, it is a consequence of the intrinsic competition for electron donor in soils and sediments, driven by a variety of electron acceptors such as nitrate, Fe, sulfate, and bicarbonate. Electron donor competition was recognized early on as an important phenomenon that needed to be characterized in order to predict.