This contributed volume explores how plant growth-promoting rhizobacterias (PGPRs) provide a wide range of benefits to the plant. Further, it discusses the key roles PGPRs play in nutrient acquisition and assimilation, improved soil texture, secreting, and modulating extracellular molecules. The book outlines how plant secondary metabolites are natural sources of biologically active compounds used in a wide range of applications, and surveys the significant role of volatile organic compounds (VOCs) in plant communication by mediating above- and below-ground interactions between plants and the surrounding organisms. This volume compiles research from leading scientists from across the globe, linking the translation of basic knowledge to innovative applied research. The book focuses on the following three categories: 1) understanding the secondary metabolites produced by PGPRs, the signaling mechanisms and how they affect plant growth, 2) the plausible role of volatile organic compounds produced by PGPRs, their role and the signaling mechanism for plant growth promotion, and 3) Applications of VOCs and secondary metabolites of PGPRs for seed germination, plant growth promotion; stress tolerance and in-plant health and immunity.