Publisher Summary The conversion of the inert nitrogen molecule into combined nitrogen that can be utilized as a mineral nutrient is brought about either by reduction to ammonia or oxidation to nitrate. This conversion, also referred to as fixation, is highly energy consuming. This is the main focus of this chapter. The capability of biological fixation of atmospheric nitrogen (N2) is restricted to organisms with prokaryotic cell structure: namely, bacteria and blue-green algae (cyanobacteria). As industrial N2 fixation requires both high temperature and pressure, the question arises as to how this reaction can proceed in living cells growing at low temperatures and atmospheric pressure, and the biochemistry of N2 fixation explains this. The discussion of symbiotic systems, therefore, focuses on legumes where the knowledge of the functioning of the system is also most advanced. In both the legume and nonlegume systems, symbiosis is characterized by a more or less distinct host preference or even host specificity. The first step of host plant infection by the microsymbiont requires recognition of the host. Of the mineral nutrients, very often combined N2 (nitrate, ammonium, urea, amino acids), both in soils and in plants, has the most prominent influence on biological N2 fixation. Inherent differences in biomass production and environmental factors, such as low and high soil fertility, contribute to this variability; an example of the wide range obtained is given for some tropical and subtropical legumes.