AbstractA discussion of the properties of that class of materials known as ‘hydrogels’ must make particular reference to (i) a working definition of what is meant by a ‘gel’, (ii) the manner in which this relates to the underlying macromolecular structure, and (iii) how these properties are affected by the presence of water within the macromolecular network. The first point is answered by providing an operational definition of a gel, which depends on the mechanical response of the material to oscillatory shear. This type of behaviour is shown to be quantitatively related to the degree of connectivity of the network. The chemical aspects of the problem address the relationship between this and the degree of reaction achieved by those chemical or physical features responsible for crosslinking. Finally, the swelling properties of such materials are discussed, and how these are complicated by the non‐ideal thermodynamic properties of the solv