This chapter focuses on the ecologically and agriculturally most important beneficial plant–microbe endosymbiotic systems, such as arbuscular mycorrhiza (AM) and rootnodule (RN) symbiosis, or root nodules. These are characterized by the formation of special complex symbiotic compartments, as well as by interactions with plant growthpromoting rhizosphere bacteria (PGPR) and/or beneficial endophytic types, where from the outset of interaction special symbiotic structures are not formed (Fig. 1). These groups of beneficial microbes improve host mineral nutrition, acquisition of water, promote plant development and offer protection from pathogens and pests. The taxonomy of the partners, the processes of the formation of symbioses and their functions will be reviewed. Development of symbiotic root nodules will be described using pea (Pisum sativum L.) as an example of an evolutionary advanced model of nodulation. Special attention will be given to developmental genetics, evolution of the symbioses and their inter-