Roots are among the most important organisms in the soil ecosystem, and the soil environment surrounding the roots is designated as the rhizosphere. The word rhizosphere has been derived from Greek, meaning the in¬uence of a root on its surrounding (Pinton and Varanini, 2001). It was ‰rst introduced by the German scientist Hiltner in 1904 to describe the interaction between microorganisms and legume plant roots. Now the term includes all plants and is a topic of fundamental importance in crop production. The interaction between microorganisms and plant roots may be positive, negative, or neutral. Hiltner observed that microorganisms were much higher in the soil surrounding plant roots than that remote from the root. He designated this zone of soil in which microorganisms were affected by plant root as the rhizosphere (Rovira and Davey, 1974). Further, Rovira and Davey (1974) reported that colonization of the rhizosphere by microorganisms varied from plant species to species (Table 4.1). In the past decades, several de‰nitions of the rhizosphere have been presented. According to the Soil Science Society of America (2008), the rhizosphere can be de‰ned as the zone of soil immediately adjacent to plant roots in which the kinds, numbers, or activities of microorganisms differ from that of bulk soil. Bowen and Rovira (1999) de‰ned the rhizosphere as the soil adjacent to roots with a different physical, chemical, and biological environment from bulk soil. Hinsinger (1998) and Hinsinger et al. (2009) de‰ned the rhizosphere as the volume of soil in¬uenced by root activity. As stressed by Hinsinger et al. (2005) and Gregory (2006), depending on the activity that one considers (exudation of reactive compounds, respiration, uptake of more or less mobile nutrients and water), the radial extension of the rhizosphere can range from submicrometer to supra-centimeter scales. Mengel et al. (2001) de‰ned that the part of the soil that is directly in¬uenced by roots and is within 1-3 mm from the root surface is called the rhizosphere. Neergaard and Magid (2001) reported that the measured extension of ryegrass (Lolium perenne L.) rhizosphere is 1-3 mm, whereas other researchers estimate that it ranges from 2 to 5 mm (Dijkstra et al., 1987; Youssef et al., 1989; Yeates and Darrah, 1991; Badalucco et al., 1996). Pepper and Bezdicek (1990) reported that the rhizosphere can extend to 20 mm as a series of gradients of organic substrate, microorganisms, pH O2, CO2, and H2O. According to Darrah (1993), the inner boundary of the rhizosphere is not better de‰ned.