Bacterial cells present a wide diversity of saccharides that decorate the cell surface and help mediate interactions with the environment. Many Gram-negative cells express O-antigens, which are long sugar polymers that makeup the distal portion of lipopolysaccharide (LPS) that constitutes the surface of the outer membrane. This review highlights chemical biology tools that have been developed in recent years to facilitate the modulation of O-antigen synthesis and composition, as well as related bacterial polysaccharide pathways, and the detection of unique glycan sequences. Advances in the biochemistry and structural biology of O-antigen biosynthetic machinery are also described, which provide guidance for the design of novel chemical and biomolecular probes. Many of the tools noted here have not yet been utilized in biological systems and offer researchers the opportunity to investigate the complex sugar architecture of Gram-negative cells.