Electron Paramagnetic Resonance (EPR) is certainly the first and the most widely used spectroscopic technique for studying structure and function of Mo and W enzymes. Although only Mo(v) and W(v) states can be detected, a considerable wealth of data was provided since the seminal EPR works performed on xanthine oxidase and nitrate reductase more than 55 years ago. In this chapter, we give a comprehensive overview of the various applications of EPR on the ubiquitous Mo-enzymes, which exhibit such an extraordinary diversity of substrates and catalyzed reactions. Elucidating the nature of Mo(v) intermediates is a considerable challenge to progress in understanding these processes. The g-tensor analyses are helpful in that aim. But it is essentially thanks to the advances in pulsed EPR methods like ENDOR, ESEEM and HYSCORE, combined with efficient isotopic enrichment strategies, that the measurements of hyperfine couplings of Mo-cofactor with neighbouring magnetic nuclei have brought the most interesting data. Thus, we illustrate how the analysis of hyperfine parameters associated with computational chemistry methods is becoming a powerful way to provide high-resolution structural data on Mo(v) species and enzyme mechanisms. In addition, EPR study of spin–spin couplings between Mo-cofactor and other paramagnetic centres appears as a promising way to gain long-range structural data in these systems.