This book provides a critical review of the equilibrium elastic properties of rubber, together with the kinetic-theory background. It is suitable for the non-specialist and the emphasis is on the physical reality embodied in the mathematical formulations. Polymer science had developed greatly since the second edition of this text in 1958, and the two main advances - the refinements of the network theory and associated thermodynamic analysis, and the development of the phenomenological or non-molecular approach to the subject - are both reflected in the structure of this third edition.