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...