Many biomaterials use metal ions to achieve high performance. This chapter will review different metal‐based cross‐linking mechanisms, then focus on their application to glue from the terrestrial slug Arion subfuscus. Metal ions can form cross‐links by bridging polymers directly. This can occur through electrostatic interactions or coordinate covalent bonds. The nature of the metal and ligand governs the stability of the linkage. Metals such as iron and copper can also catalyze oxidation reactions. These reactions can produce carbonyl groups, which react readily to form covalent linkages. Both direct and oxidatively‐derived cross‐links strengthen A. subfuscus glue. This glue is a remarkably tough adhesive gel produced in response to threats. Large, polyanionic carbohydrates are likely joined together directly by calcium ions, and possibly iron and manganese. Metal‐binding proteins may also participate in such direct cross‐links. While calcium is much more prevalent, the greater affinity of iron and manganese for the same ligands may contribute to their importance. In addition to direct cross‐links, oxidatively‐derived cross‐links strengthen the glue. Common amino acids such as lysine are oxidized to create carbonyl groups, which can react with nucleophilic side chains to form imine bonds. The combination of a deformable network of giant polysaccharides and a rigidly cross‐linked protein network could work synergistically to enhance glue toughness via a double‐network mechanism. This can produce far greater performance than would be expected for a singly cross‐linked gel. Overall, the versatile cross‐linking ability of metal ions combined with multiple types of polymers can create sophisticated, high‐performance gels.