With sufficient amount of solar energy and water in agricultural production, main factor for achieving optimum yield and for growth and development of plants, is nitrogen. Largest stock of nitrogen is in the atmosphere, where the nitrogen is found in molecular form which is unavailable for plants. Through nitrogen fixation, molecular nitrogen is reduced to compounds which plants can uptake and use for their own metabolic needs. Nitrogen fixation can be divided on symbiotic, associative and asymbiotic fixation. The most important of all, symbiotic fixation, occurs as a result of symbiotic interaction between most of leguminous plants and rhizobia. Associative nitrogen fixation is active on the root surface, and the most important bacterial genera for this type of fixation are Azotobacter, Azosirillum, Bacillus i Klebsiella. PGPR (Plant growth-promoting rhizobacteria) are free living bacteria in the soil and they have a positive impact on the plant growth. They are characterized as bacteria that promote plant growth through their metabolic activity. Current knowledge about side effects of using different chemicals in agriculture, made a huge interest for better understanding of microbial activity in the rhizosphere and its use in agriculture. Activity of microbial population can be used as an economical biotechnology and it presents the base of strategy that helps developing sustainable agricutlure and protects the environment. In sustainable agriculture, interaction between plants and microorganisms has an increassing importance in transformation and mobilization of nutrients from limited supplies in the soil, and also in uptaking those nutrients for plants to achieve their maximum genetical potential.