Natural rubber is the prototype of an important class of materials consisting of long flexible molecules which interact with each other in a particular way. Some of the properties of bulk rubber are strikingly similar to, and may be understood by a consideration of, those of single flexible molecules. For a complete understanding of the behavior of rubberlike materials it is necessary to understand the way in which they are built up from the component flexible molecules. In vulcanized materials, in which plasticity is suppressed, intermolecular bonds link the molecules into a coherent network, very irregular in detail but isotropic and homogeneous on the average. In lightly vulcanized materials these bonds are relatively few, and bring relatively small portions of adjacent molecules into fixed relations to each other. For the most part the interaction of neighboring molecules in the material is that characteristic of liquids. It is the presence of the intermolecular bonds, which link the molecules into a network and thus control its form, which differentiates rubberlike materials from liquids. It is the small number of these bonds, and their weak control of the form of material through the entropy rather than the internal energy, which differentiates rubberlike materials from ordinary solids. On the basis of this picture of the structure of rubber there is derived a form for the stress-strain curves at moderate extensions which is in good agreement with experiment.