Microorganisms are the key components of the soil biodiversity. Free-living soil bacteria beneficial to plant growth, usually referred to as plant growth-promoting rhizobacteria (PGPR), are capable of promoting plant growth by colonizing the plant root. PGPR are associated with the rhizosphere, which is an important soil ecological environment and plant health for plant–microbe interactions. Symbiotic nitrogen-fixing bacteria, viz. Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium, and Mesorhizobium, and free-living nitrogen-fixing bacteria or associative nitrogen fixers, viz. Azospirillum, Enterobacter, Klebsiella, and Pseudomonas, have been shown to attach to the root and efficiently colonize root surfaces. PGPR have the potential to contribute to sustainable plant growth promotion. Due to increase inputs of pesticides and fertilizers, the role of these microorganisms is marginalized in conventional agricultural, leading to loss of biodiversity as well as its function. However, increased awareness in many countries, including India, is progressively leading to a development from conventional intensive agriculture to sustainable agriculture. Sustainable agriculture refers to farming systems where the use of mineral fertilizers and pesticides is restricted. These agro-ecosystems are consequently more dependent upon biological control of pests and organic fertilizers to maintain crop health and productivity. In this chapter, PGPR role has been discussed in the process of plant growth promotion, their mechanisms, and their importance in crop production on sustainable basis.