Chapter One introduces metalloenzymes and focusses on hydrogenase enzymes, which catalyse hydrogen production. Specific attention is given to the [NiFe] hydrogenases including their structures, redox states and possible catalytic mechanisms. Synthetic analogues of the active site of this enzyme reported in the literature are reviewed, including Ni thiolate complexes, [NiFe] complexes and macrocyclic complexes. The aims of this project conclude the chapter.