The aerobic bacteria capable of obtaining energy from the oxidation of H2 form a heterogenous group that includes both facultative and obligate chemolithotrophs and representatives of both gram-negative and gram-positive genera. H2-oxidizing aerobes inhabit such diverse biotypes as soil, oceans, and hot springs. The oxidation of H2 in these bacteria is catalyzed by [NiFe] metalloenzymes called hydrogenases. The hydrogenases studied so far belong to two families: dimeric, membrane-bound enzymes (MBH) coupled to electron transport chains and tetrameric, cytoplasmic NAD-reducing enzymes (SH). Ni2+ is an essential component of the active site contained in the large subunit of the MBH enzymes. The genes for the MBH enzymes are located in conserved clusters of accessory genes, some of which encode maturation functions and hydrogenase-related redox proteins. Maturation of both types of hydrogenase is apparently complex, involving specific nickel incorporation and proteolytic processing steps. In Alcaligenes eutrophus and Rhodobacter capsulatus, hydrogenase expression is regulated by transcriptional activators belonging to the response-regulator family.