Gram-negative bacterial lipopolysaccharides (LPSs) have multiple roles in plant-microbe interactions. LPSs contribute to the low permeabilities of bacterial outer membranes, which act as barriers to protect bacteria from plant-derived antimicrobial substances. Conversely, perception of LPSs by plant cells can lead to the triggering of defence responses or to the priming of the plant to respond more rapidly and/or to a greater degree to subsequent pathogen challenge. LPSs are thus key molecules in the interactions between bacteria and plants, either in symbiosis or pathogenesis. Since LPSs are glycoconjugates genetically and chemically consisting of three different molecular regions, their detailed structure elucidation is a very topical and major scientific task for chemists, and is achieved by a combination of state-of-art chemical and spectroscopic techniques. Knowledge of LPSs' chemical structures is an important prerequisite for any further understanding of the biological processes in plant-microbe interactions. Moreover, the LPSs from Gram-negative bacteria - especially those originating from plant-associated bacteria - are a great source of novel monosaccharides with unusual and occasionally astounding chemical structures, never found in the eukaryotic world. This review presents the structures of LPSs from plant-associated bacteria isolated and identified from 2001 onwards. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)