Plant growth-promoting rhizobacteria (PGPR) have gained worldwide importance and acceptance for their agricultural benefits through the application of combinations of different mechanisms of action, which allows increases in crop yield. This is due to the emerging demand for reduced dependence on synthetic chemical products and to the growing necessity of sustainable agriculture within a holistic vision of development and environmental protection. The use of selected plant-beneficial rhizobacteria may represent an important biotechnological approach to alleviate the negative effects of stress and to optimize nutrient cycling in different crops. Recent progress in our understanding of their action mechanisms, diversity, colonization ability, formulation, and application should facilitate their development as reliable components in the management of sustainable agricultural systems. In addition, numerous studies indicate increased crop performance with the use of these microorganisms. In this chapter, an understanding of the direct and indirect mechanisms of action of PGPR and their various benefits to plants are summarized and discussed.