Microbial biodiversity comprises microorganisms belonging to all kingdoms: from prokaryotes (archaea and bacteria) to eukaryotes (fungi, microalgae, molds, yeasts and protists). Microorganisms make up a large part of the earth__ampersandsignrsquo;s biomass, are extraordinarily diverse and are widespread in all habitats. More than two-thirds of the total biodiversity consists of bacteria, while archaea and eukaryotes occupy less than one-third. Microorganisms interact with each other and with the biotic and abiotic components of their environment, creating ecosystems in which there is a dynamic balance between the different components. The rhizosphere is the portion of soil surrounding the roots of plants, from which they absorb the essential nutrients and water they need to grow. In addition to the roots, there are further biotic components in the rhizosphere, such as symbiotic microorganisms, beneficial and pathogenic bacteria, and microscopic and macroscopic fungi. The aim of this review is to increase the knowledge about the interactions between plants and soil microorganisms.