Abstract Rhizobium–legume symbioses are of great ecological and agronomic importance, due to their ability to fix large amounts of atmospheric nitrogen. These symbioses are characterized by the formation on legume roots of differentiated organs called nodules, in which the bacteria reduce nitrogen into ammonia used by the host plant. The early stages of the establishment of this symbiosis result from an exchange of signals between rhizobia and hosts plant: flavonoid compounds secreted by the plant roots induce the expression of rhizobial nodulation genes, which control the synthesis by rhizobia of lipooligosaccharidic compounds called Nod factors (NFs). NFs play a crucial role in nodule organogenesis and infection and in the control of the specificity of the plant–microbe interaction. Perceived by specific receptors, NFs induce in epidermal and cortical root cells complex signaling pathways leading to the coordinated development and infection of the nodule. Interestingly, components of the NF signaling pathway are also required for the establishment of a symbiosis with endomycorrhizal fungi.