Nitrogenase is the enzyme responsible for all biological nitrogen fixation, catalyzing the reduction of dinitrogen (N2) to ammonia (NH3). The best studied and most widespread nitrogenase contains a Mo cofactor called FeMo-co. Here, we focus on three aspects of nitrogenase catalysis. (i) We review the current state of knowledge about how electrons are delivered and accumulated to achieve the challenging reduction of N2. This includes a detailed view of the roles of ATP hydrolysis in the reaction. (ii) A brief description is presented of a recent study of alternative substrates that supports the proposal that nitrogenase carries out nitrogen fixation through an "Alternating Pathway". (iii) Building on recent insights into how electrons and protons are accumulated on FeMo-co, we review the studies that establish the obligatory evolution of an H2 for each N2 reduced. The resulting reductive-elimination/oxidative addition model for how FeMo-co is activated for N2 binding and reduction is described.