Gellan gum, a microbial polysaccharide, consisting of tetra-saccharide unit, glucose, glucuronic acid, glucose and rhamnose, forms a transparent gel which is heat-resistant in the presence of divalent cations. Since 1989, the collaborative research group was organised in the Research Group of Polymer Gels affiliated to the Society of Polymer Science, Japan, consisting of various laboratories with different disciplines to clarify the mechanism using the common purified sample. This special issue contains 19 papers on the molecular conformation, gel-sol transition, interaction of gellan and water, cations and sugar, based on rheology, NMR, ESR, DSC, light scattering, osmotic pressure, small angle x-ray scattering, dielectric measurement, atomic force microscopy and the industrial application of gellan gum presented at the 4th International Conference on Hydrocolloids - OCUIS 98 by the collaborative group members and by international experts.