Endophytic prokaryotes can invade the tissue of the host plant without triggering defense reactions or disease symptoms. Instead, they promote the growth of the host plant due to their ability to fix atmospheric dinitrogen and/or to produce plant growth-promoting substances. This Microbiology Monographs volume presents up-to-date findings on the interactions between plants and beneficial prokaryotes, including the use of genomics for the analysis of plant-prokaryote symbioses and their evolution. Rhizobia-legume, actinorhizal and cyanobacterial symbioses are presented.