Polymer network materials are widespread in their occurrence and application. They include surface coatings, engineering polymers, elastomers and gels. There are innumerable examples of polymer network materials in everyday use, for example gelatin, contact lenses, paints, rubber articles and car bumpers. The special characteristic of a polymer network material is a molecular structure which permeates the whole sample. Some networks occur in nature and some are synthetic, with some having a covalent molecular network structure and some attaining their network structure by the physical association of simpler molecules. In comparison with those of linear polymers, the properties of polymer networks depend to a larger extent on the methods and conditions of preparation, i.e, the properties depend not only on the chemical structure of individual polymer chains but on how those chains are connected together to form a network. This new book emphasizes the fundamentals of polymer network formation, structure and properties, and how these three aspects are interrelated and can be used to give a unified basis for understanding the behaviour of network materials.