It has been estimated that annually over 100x106 tons of atmospheric nitrogen is introduced into the global nitrogen cycle as a result of biological nitrogen fixation (BNF). Until 90s BNF was the largest source of biologically available nitrogen in the biosphere. The assessments performed in the last decade of the XX century indicate that the amounts of anthropogenically fixed nitrogen exceed those of fixed by the biological process. Ability to fix atmospheric nitrogen is quite common in nature but it occurs only within prokaryotic microorganisms. Different physiological groups of bacteria are engaged in this process. From ecological point of view there are: free-living N2 fixers (Azotobacter, Clostridium, Anabaena, Nostoc), bacteria (Azospirillum) that fix atmospheric N in close associations with roots of some plants and bacteria fixing N2 in symbiotic systems with plants (Rhizobium - legumes or Frankia - alder symbioses). In this review different symbiotic systems have been characterised as well as their ecological and agronomic importance, particularly in organic (ecological) agriculture.