Soil bacteria living around plants exert neutral, benefi cial, or detrimental effects on plant growth and development. These effects are the result of signal exchange in which there is a mutual recognition of diffusible molecules produced by the plant and microbe partners. Understanding the molecular signalling network involved in microbe–plant interaction is a promising opportunity to improve crop productivity and agriculture sustainability. Many approaches have been used to decipher these molecular signals, and the results show that plants and microorganisms respond by inducing the expression of, and releasing, a mixture of molecules that includes flavonoids, phytohormones, pattern recognition receptors, nodulins, lectins, enzymes, lipo-chitooligosaccharides, exopolysaccharides, amino acids, fatty acids, vitamins, and volatiles. This chapter reviews current knowledge of the diverse signalling pathways that are turned on when plants interact with beneficial microbes, with emphasis on bacteria belonging to the genera Rhizobium , Azospirillum , and Pseudomonas.