Biofertilizers are important components of integrated nutrient management. They are cost effective, ecofriendly and a renewable source of plant nutrients to supplement chemical fertilizers in sustainable agricultural systems in India. This book is written with the objective of covering the basic issues in Biological Nitrogen Fixation (BNF), such as physiology, biochemistry and molecular genetics of nitrogen fixation, role of signal molecules and host gene expression in nodulation and nitrogen fixation for a thorough understanding of these processes in symbiotic nitrogen fixing systems and the possibilities of extending these agronomically potential and significant processes to non-legumes. The potential benefits from the N2 fixing symbiotic systems such as Sesbania rostrate Azolla, and free-living cyanobacteria to rice crop and associative symbiotic N2 fixer Azospirillum to rainfed crops have been discussed in detail. Immobilization of cyanobacteria in a solid matrix such as polyurethane foam for maximising ammonia production in rice fields and endophytic nitrogen fixation in wheat have also been included which are considered as potential technologies in the future. The solubilization and mobilization of nutrients by phosphobacteria and VA mycorrhiza and their role as bioinoculants, Acetobacter diazotrophicus as a novel biofertilizer for sugarcane and the cycad-cyanobacterial symbiosis have been clearly elucidated. These potential biological fertilizers would play key roles in productivity and sustainability of soil and also protect the environment as ecofriendly and cost effective inputs for the farmers.