SUMMARY Lipopolysaccharides (LPS) are ubiquitous, indispensable components of the cell surface of Gram-negative bacteria that apparently have diverse roles in bacterial pathogenesis of plants. As an outer membrane component, LPS may contribute to the exclusion of plant-derived antimicrobial compounds promoting the ability of a bacterial plant pathogen to infect plants. In contrast, LPS can be recognized by plants to directly trigger some plant defence-related responses. LPS also sensitize plant tissue to respond more rapidly or to a greater extent to subsequently inoculated phytopathogenic bacteria. Sensitization is manifested by an accelerated synthesis of antimicrobial hydroxycinnamoyl-tyramine conjugates, in the expression patterns of genes coding for some pathogenesis-related (PR) proteins, and prevention of the hypersensitive reaction caused by avirulent bacteria. The description at the molecular level of the various effects of LPS on plants is a necessary step towards an understanding of the signal transduction mechanisms through which LPS triggers these responses. A definition of these signal transduction pathways should allow an assessment of the contribution that LPS signalling makes to plant disease resistance in both natural infections and biocontrol.