Publisher Summary Biological nitrogen fixation is the enzymic reduction of atmospheric nitrogen to ammonia. Along with the scientific interest of nitrogen fixation as a fundamental biochemical reaction, it is also of great ecological and agricultural importance because it is the most important source of the metabolizable nitrogen needed by all living organisms. Nitrogen fixation is catalyzed by nitrogenase, which requires energy in the form of ATP and a biologically strong reductant for the formation of ammonia. Moreover, the chapter also gives a general review of the field of nitrogen fixation with emphasis on recent developments. The study of the biochemical genetics of nitrogen fixation can yield important basic knowledge regarding the pathway, mechanism, and regulation of this process. Furthermore, in symbiotic nitrogen fixation, both plant and host bacterium carry genetic information relevant to this process, making the genetics of the system complex. Finally, understanding of the biochemical relationships in symbiotic nitrogen fixation might allow the extension of this process to non-leguminous agricultural crops.