Lipopolysaccharides (LPSs) are complex glycolipids found in the outer membrane of Gram-negative bacteria. The lipid A-core component of the LPS molecule provides a versatile anchor to which a surface polymer:lipid A-core ligase enzyme can attach one or more structurally distinct surface polymers in a single bacterial strain. In some cases the same polymer can be found on the cell surface in both lipid A-core-linked and -unlinked forms. Analysis by SDS-PAGE of populations of LPS molecules extracted from bacterial cells indicates that there is extensive heterogeneity in their size distribution. Much of the heterogeneity results from complex modal distributions in the chain length of the polymers which are attached to lipid A-core. This is the result of preferential ligation of polymers with specific degrees of polymerization during the assembly of the LPS molecule. The surface architecture of the Gram-negative bacterial cell is therefore profoundly affected by the activities of the surface polymer:lipid A-core ligase and by molecular determinants of polymer chain length. Because of the involvement of cell-surface polymers in interactions between pathogenic bacteria and their hosts, these enzymatic activities also have an important impact on virulence. In this review, the organization of LPSs and related surface polymers will be described and the current understanding of the molecular mechanisms involved in surface diversity will be discussed. Emphasis is placed on the Enterobacteriaceae, but similarities to other bacteria suggest that aspects of the enterobacterial system will have broader significance.