Even today, more than 100 years after the invention of the Haber-Bosch process for industrial generation of NH3, biological nitrogen fixation — the reduction of N2 to two NH3 molecules — supports more than half the human population. This process, which involves one of the most challenging chemical transformation in biology, the reduction of the N≡N triple bond, is catalyzed by nitrogenases, primarily the Mo-dependent enzyme. Despite decades of intensive study in many laboratories, until recently surprisingly little was known about the mechanism of this process. Enzymatic measurements, advanced paramagnetic resonance (EPR/ENDOR), photophysical, bioelectrochemical, and computational studies of intermediates formed during turnover of the wild-type enzyme have now embedded the previously mysterious properties of nitrogenase into the fabric of inorganic/organometallic chemistry, thereby revealing the central mechanistic steps by which nitrogenase is activated to carry out NH3 formation.