Hydrogenases catalyse the reversible reduction of protons to molecular hydrogen. Applied research is focused on structure and catalytic function under the aspect of hydrogen formation. In this review we summarize the current knowledge about properties and physiological roles of hydrogenases in pro- and eukaryotes and compile molecular genetical data about structural features of prokaryotic hydrogenases. Finally, prospects are given for the possible application of hydrogenases or ’hydrogenase-like catalysts’ in energy production.