In this review, an appraisal of the progress in understanding of the NiFe-hydrogenases catalytic mechanism is presented. The article is focussed on NiFe-hydrogenases from Desulfovibrio genus, as these are among the best-characterized H 2 -splitting enzymes. The X-ray structures, catalytic reactivity, spectroscopic properties and computer modeling data of the different forms of this class of hydrogenase are analyzed. In addition, the contribution of site-directed mutagenesis studies of Desulfovibrio fructosovorans species to structure–function relationships is reviewed.