Nitrogenase catalyzes the enzymatic reduction of atmospheric dinitrogen to ammonia during the process of biological nitrogen fixation. Nitrogenase consists of two component metalloproteins, the iron (Fe‐) protein and the molybdenum‐iron (MoFe‐) protein, that together mediate the ATP‐hydrolysis–dependent reduction of substrates to products. Crystallographic studies have established the structures of the component proteins and the associated complex metallocenters of nitrogenase, including the iron‐molybdenum cofactor that provides the active site for substrate reduction and the P‐cluster that participates in electron transfer between the Fe‐protein and MoFe‐protein. Striking parallels are evident in the interaction of the nucleotides with the Fe‐protein and with a broad class of nucleotide‐binding proteins involved in signal and energy transduction processes. Together with kinetic, spectroscopic, and synthetic model compound studies, these structures provide a framework for addressing the mechanism of substrate reduction by nitrogenase.