This is the self-contained fifth volume of a comprehensive seven-volume series covering both fundamental and applied aspects of nitrogen-fixation research since the 19th century, addressing associations less formalized than the rhizobia-legume asociatons but have a myriad of effects on their plant hosts. The book describes milestones in the discovery of the associative and endophytic nitrogen-fixing bacteria found involved with cereal crops, forage grasses, and sugar cane. It provides a comprehensive overview of their phylogeny, physiology, and genetics as well as of the biology of their association with their host plants, including tools for in situ localization and population-dynamics analysis. Also included are chapters describing the functions required for a bacterium to be competent and competitive in the rhizosphere, and analysis of associations of cyanobacteria with fungi, diatoms, bryophytes, cycads, Azolla, and Gunnera, including the complex regulatory network that controls the differentiation of vegetative cells into nitrogen fixing heterocysts.