The active sites of the multiheme protein cytochrome c nitrite reductase (NiR) and of the copper enzyme nitrous oxide reductase (N2OR) have been studied by various spectroscopic techniques including continuous wave multifrequency and multiquantum electron paramagnetic resonance (EPR), electron spin echo envelope modulation (ESEEM), electron nuclear double resonance (ENDOR), saturation recovery (SATREC), linear electric field effect (LEFE), magnetic circular dichroism (MCD), and X-ray absorption spectroscopy (XAS/EXAFS). In addition, calculations at the iterative Extended Hueckel and UHF-INDO/S level were performed to obtain MO diagrams for the mixed-valence CuA center in N2OR which help to understand the unusual magnetic and electronic properties of this novel chromophore. Based on the spectroscopic and biochemical properties of both NiR and N2OR, structural models for their active sites have been developed.