Hydrogenases are enzymes found in all domains of life that catalyze a remarkably simple chemistry, the reversible oxidation of molecular hydrogen to protons and electrons. In order to perform this chemistry, cells have evolved, several different times, intricate organometal complexes built around a binuclear Ni-Fe or Fe-Fe center, with bound CO and CN− groups, as well as multiple FeS centers. These complicated enzymes have been an area of intense study for many decades, with interest peaking on the occasions of major increases in national energy costs. Interest in biologically generated hydrogen as a potential substitute for fossil fuels is again at the forefront, and the new tools of the postgenomic world available for manipulating these enzymes make it a truly viable possibility. Hydrogenases from hyperthermophilic microorganisms such as Pyrococcus furiosus and Thermotoga maritima, with optimal growth temperatures near 100°C, are of particular interest and promise for elucidating and manipulating these enzymatic mechanisms.