Nitrite reductase (NIR) is a key enzyme in the dissimilatory denitrification chain, catalyzing the reduction of NO2 - to NO. Although a matter of debate for a long time, NO is now accepted product of NO2 --reduction as an obligatory intermediate in most denitrifiers, and it is further reduced to N2O by NO reductase. Purification and characterization of NIR from several bacterial sources have shown that there are two distinct classes of dissimilatory NIRs which yield NO as the main reaction product, containing either copper (CuNIR) or heme (cd1NIR) as cofactor, the heme containing enzyme occurring more frequently. The genes coding for the CuNIR and cd1NIR apoproteins are called nirK and nirS, respectively. Besides the species from which the enzyme has been purified, several others were shown to contain one of the two types of NIR on the basis of DNA hybridization and/or inhibition of denitrification by the Cu chelator diethildithiocarbamate (DDC). The enzymes containing Cu and heme never coexist within the same bacterial species. Functional complementation of a cd1NIR deficient strain of Pseudomonas stutzeri with the Cu NIR from Pseudomonas aureofaciens indicates that the two enzymes fulfil the same role in vivo. This chapter mainly focuses on the structure-function relationships in the two classes of dissimilatory NIRs and special attention is paid to recent structural information on enzymes from different sources, which have different structures and catalyse the reduction of NO2- to NO via different mechanisms.