Technical Abstract: This paper reviews the structural organization of polysaccharides with respect to molecules, clusters (aggregates), networks and interactions. As for proteins, different levels of structural organization exist for polysaccharides. The primary structure describes the covalent sequence of monosaccharide residues in the polymer chain; the secondary "ordered" structure defines any geometrically regular arrangement in space that this sequence may adopt; the tertiary structure defines the way these arrangements pack together and higher quarternary structures mostly define polysaccharide-polysaccharide interactions. Up-to-date experimental techniques to characterize secondary and higher structural organization are described. Polysaccharide clusters and gel networks are illustrated with carrageenan, alginate, pectin, and gellan. Higher levels of structural organization (polysaccharide-polysaccharide interactions), are discussed for the synergistic interactions of galactomannans or konjac glucomannans with helix-forming algal polysaccharides or xanthan. Finally the future outlook for the fast growing area of the three-dimensional structure of polysaccharides at the molecular level is discussed.