Hydrogenase enzymes catalyze the rapid and reversible interconversion of H-2 with protons and electrons. The active site of the [FeFe] hydrogenase is the H cluster, which consists of a [4Fe4S](H) subcluster linked to an organometallic [2Fe](H) subcluster. Understanding the biosynthesis and catalytic mechanism of this structurally unusual active site will aid in the development of synthetic and biological hydrogenase catalysts for applications in solar fuel generation. The [2Fe](H) subcluster is synthesized and inserted by three maturase enzymesHydE, HydF, and HydGin a complex process that involves inorganic, organometallic, and organic radical chemistry. HydG is a member of the radical S-adenosyl-l-methionine (SAM) family of enzymes and is thought to play a prominent role in [2Fe](H) subcluster biosynthesis by converting inorganic Fe2+, l-cysteine (Cys), and l-tyrosine (Tyr) into an organometallic [(Cys)Fe(CO)(2)(CN)](-) intermediate that is eventually incorporated into the [2Fe](H) subcluster. In this Forum Article, the mechanism of [2Fe](H) subcluster biosynthesis is discussed with a focus on how this key [(Cys)Fe(CO)(2)(CN)]- species is formed. Particular attention is given to the initial metallocluster composition of HydG, the modes of substrate binding (Fe2+, Cys, Tyr, and SAM), the mechanism of SAM-mediated Tyr cleavage to CO and CN , and the identification of the final organometallic products of the reaction.