Azospirillum is a plant growth promoting rhizobacterium used for inoculation of cereal and forage crops. The book covers its physiology, ecology, biochemistry, and molecular biology. The most advanced molecular techniques to understand the regulatory mechanisms of nitrogen fixation and ammonia assimilation, as well as the basis of phytohormone production, are included. In particular, the identification of novel types of promoters, specific regulatory circuits, and new regulatory proteins is described. New insights in the plant growth promoting role of the bacteria through the analysis of their interactions with the plant are presented. Also discussed are field applications, allowing the evaluation of the physiological and agronomic involvement of Azospirillum inoculations.