Hydrogenases are generally classified as iron-sulfur-containing proteins with four to twelve iron atoms in different cluster arrangements. Through physiological, chemical and spectroscopic studies, nickel has also been found to be a constitutive metal of several hydrogenases and shown to be involved in a redox linked process. The [NiFe] hydrogenases are now the most intensivelly studied nickel enzymes. Here we will focus on the study of [NiFe] hydrogenases isolated from sulfate reducing bacteria (Desulfovibrio (D.) species). D.gigas hydrogenase is used as a model system for the discussion of the spectroscopic features of the metal centers in the native and hydrogen reacted states. A reasonable understanding of the native enzyme metal center constitution has emerged from EPR and Mossbauer (MB) spectroscopic studies, indicating the presence of four non-interacting centers: one nickel (assigned to be in the unusual trivalent state), one [3Fe-4S] center (EPR active) and two [4Fe-4S] clusters (EPR silent). The intermediate redox species generated under H2 were studied in order to detect the redox processes involved and to determine the associated mid-point potentials. The occuring redox transitions (nickel is assumed to cycle between tri and divalent oxidation states) are related to the redox linked activation step required for the full expression of the enzymatic activity. Special emphasis is given to the interpretation of the EPR and MB results obtained in the enzyme active state and the interplay between the metal centers. The existence of a proton and a hydride acceptor sites are postulated in agreement with a heterolytic cleavage step of the hydrogen molecule. The properties of relevant nickel compounds and functional models, are used in order to define the oxidation states involved and the favored coordination of the nickel site. The assemble of the available experimental data is integrated in the enzyme mechanistic framework. Selenium is also an essential constituent of some [NiFe] hydrogenases. Its role on the fine tuning of the catalytical properties and as a proximal nickel ligand is discussed.