Hydrogenases are enzymes capable of catalyzing the oxidation of molecular hydrogen or its production from protons and electrons according to the reversible reaction: H(2)<==>2H(+)+2e(-). Most of these enzymes fall into to major classes: NiFe and Fe-only hydrogenases. Extensive spectroscopic, electrochemical and structural studies have shed appreciable light on the catalytic mechanism of hydrogenases. Although evolutionarily unrelated, NiFe and Fe-hydrogenases share a common, unusual feature: an active site low-spin Fe center with CO and CN coordination. We have recently focused our attention on Fe-hydrogenases because from structural studies by us and others, it appears to be a simpler system than the NiFe counterpart. Thus the primary hydrogen binding site has been identified and plausible, electron, proton and hydrogen pathways from and to the buried active site may be proposed from the structural data. The extensive genome sequencing effort currently under way has shown that eukaryotic organisms contain putatively gene coding sequences that display significant homology to Fe-hydrogenases. Here, we summarize the available evidence concerning the mechanism of these enzymes and carry out a structural comparison between Fe-hydrogenases and related proteins of unknown metal content from yeast, plant, worm, insect and mammals.