Rhizobia are nitrogen-fixing bacteria which invade root hairs of leguminous plants and induce, in a specific manner, the formation of root nodules in which they fix nitrogen. The early steps of the symbiosis can be considered as a reciprocal molecular communication between the two partners. Initially, the plant excretes a gene inducer which stimulates the expression of bacterial nodulation genes. These nodulation genes are responsible for the synthesis of extracellular host-specific signals, called nodulation factors. The bacterial nodulation factors were isolated and structurally identified as substituted and N-acylated chitin oligosaccharides. These prokaryotic lipo-oligosaccharide signals play a key role in the symbiosis by controlling the host specificity of the bacteria. They constitute a new class of signalling molecules able to elicit nodule organogenesis in leguminous plants in the absence of bacteria.