Methanogens and related methanotrophs are unique among archaea for containing nitrogenases, the metalloenzymes essential for biological nitrogen fixation (diazotrophy). All diazotrophic methanogens utilize molybdenum (Mo) nitrogenase, which consists of dinitrogenase reductase (NifH) with a [4Fe-S] cluster and dinitrogenase (NifDK) containing the complex P-cluster and catalytic M-cluster or FeMo cofactor. Some species also possess vanadium (V) nitrogenase and/or Fe-only nitrogenase, each composed of homologous components. In comparison to diazotrophic bacteria, the mechanisms of cofactor biogenesis and the regulation of nitrogenase production and activity in methanogens are not well understood. We are using Methanosarcina acetivorans as a model methanogen to investigate these factors. Notably, the genome of M. acetivorans encodes all three types of nitrogenase and features a robust genetic system, including CRISPR systems. Our research, employing genetic, physiological, biochemical, and structural approaches, has uncovered unique aspects of nitrogenase assembly and regulation in M. acetivorans compared to other diazotrophs. These include differences in cofactor biogenesis, the production of V-nitrogenase and Fe-nitrogenase dependent on Mo-nitrogenase expression, and the formation of unique higher-order complexes by Mo-nitrogenase that likely facilitate coordinated catalysis. Overall, our findings provide new insights into the complexity of nitrogenase utilization by methanogens that will aid efforts to engineer nitrogen fixation in non-diazotrophs and to understand the origin and evolution of nitrogenases.