This chapter reviews the microstructural evolution of single polysaccharide gels, mixed protein–polysaccharide systems and fluid gels. It also covers the physicochemical behaviour of the crystalline and glassy solid states of polysaccharide systems. The chapter outlines the structural monomers of the polysaccharide backbone and goes on to describe how the various structures are built and stabilized. The forces that act among the chains are also discussed, as they control how the structures evolve and stabilize. Two regimes, which depend on the water content of the systems, can be distinguished whereby polysaccharides form completely different structures with distinct physical and mechanical properties. In the high moisture regime polysaccharides are able to form gels, making it possible to structure water or air. At the opposite extreme, where moisture content is low, crystalline and glassy states and the related relaxation phenomena control the structural stability of these intricate carbohydrate polymers. The greatest drawback for materials based on polysaccharides is their metastability, that is, their sensitivity to structural evolution in time. In food applications this is usually manifested by limited shelf-life and changes in functional and textural properties during storage.