Various kinds of endophytic and rhizospheric micro-organisms were closely related with various plants. Most of these micro-organisms were potential promoters of plant growth and development as they produced phytohormones that increased available nutrients to plants, controlled plant pathogens, etc. These microbes were collectively defined as plant growth-promoting micro-organisms. Furthermore, volatiles released by these plant growth-promoting micro-organisms were critical for mutual symbiosis of the host plants and plant-associated micro-organisms. Many endophytic and rhizospheric micro-organisms were reported to be capable of producing varieties of volatiles as secondary metabolites. Volatiles produced by plant growth-promoting micro-organisms had many essential ecological functions and the relationships among the functions were often quite complex. Some volatiles produced by plant growth-promoting micro-organisms were capable of promoting growth and development of plants. These volatiles induced systemic resistance in plants such that survival rates of plants in negative conditions improved. Also these compounds altered the constituents and contents of valuable secondary metabolites in plants, which in turn enhanced plants biotic resistance. Finally, some volatiles inhibited growth or reproduction of pathogens or insects and prevented herbivores from grazing on the host plants. Given these functions and several more, these critical metabolites enhanced host plants' competitive advantages over non-host plants. Because of these valuable ecological functions, further research studies needed to be done on beneficial effects of mutual symbiosis of micro-organisms and host plants. Several volatiles produced by plants affected signaling pathways or gene expression of host plants. In summary, studies on volatiles produced by plant growth-promoting micro-organisms enabled us to better understand the complex relationships among plants and micro-organisms. For in-depth research on volatiles produced by plant growth-promoting micro-organisms, this review summarized the results of current studies and discussed further development in this field.