Nitrogen is a key element in the nutrition of living things because of its importance in nucleic acids and in proteins. Gaseous nitrogen is present in abundance in the Earth’s atmosphere forming about 80% of atmospheric gases. Unfortunately, most living organisms cannot utilize gaseous nitrogen but require it in a fixed form. Nitrogen can be fixed both chemically and biologically. Chemical fixation is employed in the production of nitrogenous chemical fertilizers which are used to replace nitrogen removed from the soil by plants. Biological fixation is found only in bacteria and blue-green algae. Some of these organisms fix nitrogen in the free-living state contributing to the improvement of the nitrogen status of the soil. Others do so closely associated (in symbiosis) with higher plants. In some of these associations, the organisms live on the surface of the plant roots and fix the nitrogen there. In some others, the microorganism such as Rhizobium penetrates the roots and forms outgrowths known as nodules within which nitrogen is fixed. This chapter describes the biology of Rhizobium, cross-inoculation groups of rhizobia, desirable properties in strains to be selected for use as Rhizobium inoculants, fermentation for rhizobia, inoculant packaging, seed inoculants and soil inoculants.