Nitrogen is an essential constituent of living organisms and there is an inexhaustible supply of it in the atmosphere in the free form. Majority of living organisms including humans are incapable of utilising gaseous nitrogen but need organic nitrogen for their nutrition. It is for this reason that the nitrogen fixation or availability of organic nitrogen is critical for the living organisms. Free nitrogen is inert but certain groups of specialised organisms have the ability to utilise gaseous nitrogen fmn the atmosphere. The reduction of gaseous nitrogen or its conversion to organic nitrogen is called nitrogen fixation. The fixed nitrogen is readily utilised by other organems. Incorporation of atmospheric nitrogen into nitrogen compounds by nitrogen fixers such as Azotobacter in aerated soils; Clostridium in unaerated soils; Rhizobium in the nodules of legume roots and by a number of blue green algae such as Nostoc and Anabaena, is the most important step to ensure nitrogen supply for the living organisms. The quantum of biological nitrogen-fixation has direct effect on soil fertility.