A review is presented focussing attention on the structural molecular biology of polysaccharides and complex carbohydrates, using examples obtained from terraqueous plants, animals, bacteria and insects The type and sequence of the condensation linkages in polysaccharides dominate their conformation, flexibility and interactions The extensive variety of geometries is overlaid by the constituent saccharide units themselves, decoration by side appendages and post-polymerisation chemical and structural modification X-ray diffraction information from oriented samples and computerised modelling has been used to analyse molecular conformation and geometry In general the relationship between glycosidic linkage geometry and conformation for the chemically simpler polysaccharides is understood In the case of more complex carbohydrates, unique solutions using diffraction methods alone are harder to establish In mixed protein carbohydrate systems, such as the glycoprotein antifreezes and protein-polysaccharide fibrous composites in insect cuticle, novel features in structure, morphology and interactions can usefully be explored and examined.