The lipopolysaccharides (LPS) are major components of the outer membrane of Gram negative bacteria and, because of their location, are important mediators in the interaction between these bacteria and their environment and other organisms. The α-Proteobacterial family Rhizobiaceae includes the rhizobia and agrobacteria, microorganisms which establish symbiotic or parasitic relationships with plants. Mutants deficient in LPS biosynthesis show anomalous interactions with their hosts. The agronomical relevance of the relationship between rhizobia and agrobacteria with plants has promoted a large number of studies on the LPS from these bacteria. The complete structures of one or several domains of LPS from Rhizobiaceae have been determined in the last years. Additionally, several metabolic steps in the biosynthesis of these molecules have been elucidated. This review aims at the description of the more recent findings on the structure and biosynthesis of LPS in Rhizobium, Sinorhizobium and Agrobacterium.