Polymer networks can be divided into two main classes, those formed at high segment concentrations by the topological entanglement of polymer chains and those formed formally by the covalent cross-linking of preformed linear chains. The former are networks at frequencies higher than some typical entanglement lifetime, and are viscoelastic liquids, whereas the latter are of infinite molecular weight, and have an equilibrium modulus-they are viscoelastic solids. However, there are many intermediate systems of chains which are "physically" cross-linked, the cross-links themselves being of small but finite energy, and/or of finite lifetime; these are called physical networks. Specific systems include both biological and synthetic polymers. Amongst the former are gelatin, and the seaweed polysaccharides such as agarose and the carrageenans. Such biopolymers form physical gels involving junction zones of known, ordered secondary structure. Models are presented that relate the modulus and geltime kinetics to properties of the macromolecular chains. Covalent and Entanglement NetworksPolymer gels or networks can be divided into two main classes, chemically cross-linked materials (including bulk elastomers), and "entanglement networks". The covalently cross-linked materials are formed by a variety of routes including cross-linking high molecular weight linear chains, either chemically or by radiation, by end-linking react ant chains with a branching unit, or by step-addition polymerisation of oligomeric multi-functional precursors. They are true macromolecules, where the molecular weight is nominally infinite, and they therefore possess an infinite relaxation time

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