The chemical, physical, and biological interactions between roots and the surrounding environment of the rhizosphere are some of the most complex experienced by land plants. Over the last few years the field of rhizosphere biology has recognized the biological importance of root exudates in mediating interactions with other plants and microbes (Bais et al. 2004a, b; Walker et al. 2003; Weir et al. 2004). Chiefly, root exudates comprise two different classes of compounds. Low molecular weight compounds include amino acids, organic acids, sugars, phenolics, and various secondary metabolites, whereas high molecular weight exudates primarily include mucilage (high molecular weight polysaccharides) and proteins. Root exudation clearly represents a significant carbon cost to the plant (Uren 2000); however, the molecular mechanisms regulating exudation are still poorly understood. The roots of some plants also release border cells into the rhizosphere but literature discussing this phenomenon will not be covered in this chapter (for information see Hawes et al. 2000; Vicre et al. 2005).

The rhizosphere comprises the area of soil immediately surrounding a plant root and represents a highly dynamic environment involving interactions with competing roots and pathogenic/nonpathogenic microbes and invertebrates (Hirsch et al. 2003). The focus of this chapter will be on root–microbe interactions that can be broadly divided into positive interactions including classic symbioses, association with bacterial biocontrol agents, epiphytes, and mycorrhizal fungi; and negative interactions including associations with parasitic plants, pathogenic bacteria, fungi, and invertebrate herbivores. Microbial colonization of the rhizosphere is important not only as the first step in pathogenesis of soil-borne