Plant roots are the major source of available carbon in the soil. Therefore, soil bacteria maintain adaptive traits which enable them to exploit this highly competitive niche, yet microbial associations are inherent to plant adaptation to the highly heterogeneous soil environment. The rhizosphere has been, for many years, the focus of intense basic and applied research, aimed at modulating and gaining control over this environment to promote plant health and development. Such studies have revealed the high complexity of plant-associated microbial communities and the many factors that influence root-associated bacterial community composition, including the plant species and growth stage, as well as the soil physical, chemical, and biological characteristics. The high availability of carbon promotes copiotrophic/zymogenic life style in rhizosphere microorganisms. High diversity is maintained due to the prevalence of multiple micro-niches and utilization of different competition strategies. This may be fundamental to some of the most important plant growth and health promoting effect of root-microbe associations.