Respiratory nitrate reductases (Nars) are oxidoreductases that depend on complex cofactors that contain different transition metals. Nars are produced by microorganisms that can grow under denitrifying conditions and catalyse the first step of the denitrification, a geochemical process that involves the conversion of nitrogen oxoanions and oxides into dinitrogen (N2[g]). Nars are complex enzymes with an (αβγ)2 biological assembly. The α- and β-subunits harbour the Mo-bisPGD cofactor and iron–sulphur clusters, while the integral membrane γ-subunit contains two b-type haems. The latter are involved in the oxidation of the membrane quinol pool and the generation of a proton gradient that energises the bacterium cell. In the present chapter, we update some general aspects of molybdo-enzymes and the present knowledge on Nars. Firstly, the special chemical properties that make Mo an efficient O-atom transfer catalyst are briefly presented. Next, the classification of molybdo-enzymes into different families based on the structural properties of the active site is discussed. Then, we review in detail the structural and spectroscopic properties of each subunit comprising the Nar heterotrimer, as well as the complex regulation of nar gene expression, the metabolic role of Nars within the bacterium cell and some key aspects of the catalytic mechanism.