Denitrification returns dinitrogen to the atmospheric reservoir via the reductive transformation of nitrogen oxides. It is part of the N cycle and counterbalances processes of natural chemical N fixation and bacterial N2 fixation. Understanding the molecular basis of bacterial denitrification requires knowledge about the participating enzymes, their cellular organization, and identification of the necessary genes and their control mechanisms. This knowledge will provide insights about what limits the denitrification process, how it is influenced by environmental parameters, and what results in the liberation of nitrite and the two gaseous intermediates nitric oxide and nitrous oxide.