The role of microorganisms in nitrogen fixation in legumes was discovered in the nineteenth century. The symbiosis of microorganisms in legumes is displayed as formation of nodules on stems or roots. The effective rhizobium-legume associations are determined by flavonoids. Nitrogen is the main limiting nutrients for growth and productivity of non-crop and crop species. Biological nitrogen fixation is an important mechanism like that of photosynthesis for plant growth and development. Under natural environmental conditions, plants face a combination of environmental stresses that induce substantial negative impact on plant growth and productivity. Salinity is the main constraint for plant growth and productivity in arid and semiarid areas of the world. Soil infertility due to the existence of huge quantities of salts and introduction of plants able to tolerate such constraints should be studied. Plant growth-promoting rhizobacteria strains possess important biological activities including indole acetic acid and siderphore production as well as nitrogen fixation and phosphorus solubilization.