The rhizosphere is a very complex environment. The interactions among microorganisms and the plant roots are essential for the nutrition, growth and productivity of the plant. On the other hand, the microorganisms also benefit from their interactions with the plant. Rhizosphere studies are of interest to agriculturists, soil biologists, chemists, microbiologists and molecular biologists. Microbial Activity in the Rhizosphere will be useful to the beginner as well as to experienced scientists. It starts with contributions dealing with the basic concepts in rhizosphere biology and the techniques used in these studies. Further topics include the bacterial community composition, rhizosphere of arbuscular mycorrhizal roots, significance of AM fungi in relation to form and functioning of the root system, the role of microsymbionts in plant growth and health, and the impact of rhizosphere environment on pre-infection behaviour of Oomycetous plant pathogens like Aphanomyces, Phytophthora and Pythium.