Iron represents an essential nutrient for all living organisms. It is a component of many cellular structures and also necessary for metabolic processes. Iron nutrition is particularly important for leguminous plants as it plays a vital role in nodule formation, N2-fixation and plant growth. Under iron limited conditions, some microorganisms and plants are able to evolve low molecular weight iron chelators called siderophores. These biological chelators are strongly involved in competition of iron nutrition and in biocontrol of phytopathogenic fungi by sequestering iron and thereby inhibiting their growth or metabolic activity. The results presented in this book aimed to study the interactions between siderophore producing microorganisms in the rhizosphere.