Lipooligosaccharides (LOS) are made by gram-negative bacteria that colonize mucosal surfaces other than those of the gut. The structures of these surface-exposed outer membrane organelles more closely resemble human cell membrane glycosphingolipids, with which they share glycose moieties, than the analogous lipopolysaccharides (LPS) of enteric gram-negative bacteria. Gram-negative bacteria that colonize respiratory and genital mucosal surfaces are not exposed to bile acids. The different biologies of the two sorts of glycolipids led us to propose, in 1984, that those of mucosal bacteria be called LOS, rather than lipopolysaccharides. The past decade has seen an explosion in our understanding of the structure and function of bacterial LOS, particularly those of the genus Neisseria. This chapter summarizes author current understanding of the most important bacterial organelle.