OBJECTIVE This review summarizes the spectroscopic results, which will provide useful suggestions for future research. In addition, the fields that urgently need more information are also advised. BACKGROUND Nitrite-NO-cGMP has been considered as an important signaling pathway of NO in human cells. To date, all the four known human molybdenum-containing enzymes, xanthine oxidase, aldehyde oxidase, sulfite oxidase, and mitochondrial amidoxime-reducing component, have been shown to function as nitrite reductases under hypoxia by biochemical, cellular, or animal studies. Various spectroscopic techniques have been applied to investigate the structure and catalytic mechanism of these enzymes for more than 20 years. METHODS We summarize the published data on the applications of UV-vis and EPR spectroscopies, and X-ray crystallography in studying nitrite reductase activity of the four human molybdenum-containing enzymes. RESULTS UV-vis has provided useful information on the redox active centers of these enzymes. The utilization of EPR spectroscopy has been critical in determining the coordination and redox status of the Mo center during catalysis. Despite the lack of substrate-bound crystal structures of these nitrite reductases, valuable structural information has been obtained by X-ray crystallography. CONCLUSIONS To fully understand the catalytic mechanisms of these physiologically/pathologically important nitrite reductases, structural studies on substrate-redox center interaction are needed.