The bacterial [NiFe]-hydrogenases have been classified as either 'standard' or 'O-2-tolerant' based on their ability to function in the presence of O-2. Typically, these enzymes contain four redox-active metal centers: a Ni-Fe-CO-2CN(-) active site and three electron-transferring Fe-S clusters. Recent research suggests that, rather than differences at the catalytic active site, it is a novel Fe-S cluster electron transfer (ET) relay that controls how [NiFe]-hydrogenases recover from O-2 attack. In light of recent structural data and mutagenic studies this article reviews the molecular mechanism of O-2-tolerance in [NiFe]-hydrogenases and discusses the biosynthesis of the unique Fe-S relay.