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Mining and enrichment of uranium (U) for nuclear weapons and energy has left this radionuclide an important groundwater contaminant in the United States and worldwide. The use of nitric acid in these processes has also resulted in low pH and high nitrate concentrations in U-contaminated subsurface sediments. This presents problems for bioremediation strategies to control the migration of U in groundwater, usually achieved through microbial reduction of soluble U(VI) to insoluble U(IV) upon electron donor addition to the subsurface. Nitrate, which serves as a competitive electron acceptor in the subsurface, can inhibit or retard U(VI) reduction efforts; as well, intermediates of nitrate respiration (or denitrification), such as nitrite, can lead to U(IV) oxidation and remobilization. The Integrated Field Research Challenge site in Oak Ridge, Tennessee provides an ideal location to address the challenges nitrate poses to uranium bioreduction as the site encompasses several different geochemical conditions: two acidic U- high-nitrate U-contaminated sites (both Areas 1 and 3), a low-nitrate U-contaminated site (Area 2), and a pristine (uncontaminated) background site. In this paper, we review 24 studies examining the microbial communities from these sites within the OR-IFRC as well as denitrifying fluidized bed reactors (FBRs) treating high-nitrate groundwater in an effort to describe the overall potential denitrifying community composition at these sites. Pseudomonas was the most widely detected genus among all sites, but was not detected in either of two studies describing metabolically active populations from community total RNA extracts. Collectively, 16S rRNA gene surveys indicate the following genera may be of potential importance in nitrate reduction and denitrification at the OR-IFRC: Ralstonia and Dechloromonas in the low nitrate neutral pH Area 2, Castellaniella and Burkholderia in Area 1, Thiobacillus and Ferribacterium in Area 3, and Acidovorax in FBRs. This work begins to help us understand how geochemical conditions can determine the composition of nitrate-reducing microbial communities at uranium contaminated sites as well as how population structure and physiologies of the microorganisms present affect in situ rates of denitrification and radionuclide immobilization. [ABSTRACT FROM AUTHOR]